platform: track IFA_BROADCAST address in NMPlatformIP4Address

- track the broadcast address in NMPlatformIP4Address. For addresses
  that we receive from kernel and that we cache in NMPlatform, this
  allows us to show the additional information. For example, we
  can see it in debug logging.

- when setting the address, we still mostly generate our default
  broadcast address. This is done in the only relevant caller
  nm_platform_ip4_address_sync(). Basically, we merely moved setting
  the broadcast address to the caller.
  That is, because no callers explicitly set the "use_ip4_broadcast_address"
  flag (yet). However, in the future some caller might want to set an explicit
  broadcast address.

In practice, we currently don't support configuring special broadcast
addresses in NetworkManager. Instead, we always add the default one with
"address|~netmask" (for plen < 31).

Note that a main point of IFA_BROADCAST is to add a broadcast route to
the local table. Also note that kernel anyway will add such a
"address|~netmask" route, that is regardless whether IFA_BROADCAST is
set or not. Hence, setting it or not makes very little difference for
normal broadcast addresses -- because kernel tends to add this route either
way. It would make a difference if NetworkManager configured an unusual
IFA_BROADCAST address or an address for prefixes >= 31 (in which cases
kernel wouldn't add them automatically). But we don't do that at the
moment.

So, while what NM does has little effect in practice, it still seems
more correct to add the broadcast address, only so that you see it in
`ip addr show`.

src/platform/nm-linux-platform.c

@@ -3012,6 +3012,12 @@ _new_from_nl_addr (struct nlmsghdr *nlh, gboolean id_only)
 			memcpy (&obj->ip4_address.address, nla_data (tb[IFA_LOCAL]), addr_len);
 		if (tb[IFA_ADDRESS])
 			memcpy (&obj->ip4_address.peer_address, nla_data (tb[IFA_ADDRESS]), addr_len);
+
+		_check_addr_or_return_null (tb, IFA_BROADCAST, addr_len);
+		obj->ip4_address.broadcast_address =   tb[IFA_BROADCAST]
+		                                     ? nla_get_u32 (tb[IFA_BROADCAST])
+		                                     : 0u;
+		obj->ip4_address.use_ip4_broadcast_address = TRUE;
 	} else {
 		/* For IPv6, IFA_ADDRESS is always present.
 		 *
@@ -4177,6 +4183,7 @@ _nl_msg_new_address (int nlmsg_type,
                      int scope,
                      guint32 lifetime,
                      guint32 preferred,
+                     in_addr_t ip4_broadcast_address,
                      const char *label)
 {
 	nm_auto_nlmsg struct nl_msg *msg = NULL;
@@ -4220,16 +4227,8 @@ _nl_msg_new_address (int nlmsg_type,
 	if (label && label[0])
 		NLA_PUT_STRING (msg, IFA_LABEL, label);
 
-	if (   family == AF_INET
-	    && nlmsg_type != RTM_DELADDR
-	    && plen < 31 /* RFC 3021 */
-	    && address
-	    && *((in_addr_t *) address) != 0) {
-		in_addr_t broadcast;
-
-		broadcast = *((in_addr_t *) address) | ~_nm_utils_ip4_prefix_to_netmask (plen);
-		NLA_PUT (msg, IFA_BROADCAST, addr_len, &broadcast);
-	}
+	if (ip4_broadcast_address != 0)
+		NLA_PUT (msg, IFA_BROADCAST, sizeof (in_addr_t), &ip4_broadcast_address);
 
 	if (   lifetime != NM_PLATFORM_LIFETIME_PERMANENT
 	    || preferred != NM_PLATFORM_LIFETIME_PERMANENT) {
@@ -8005,6 +8004,7 @@ ip4_address_add (NMPlatform *platform,
                  in_addr_t addr,
                  guint8 plen,
                  in_addr_t peer_addr,
+                 in_addr_t broadcast_address,
                  guint32 lifetime,
                  guint32 preferred,
                  guint32 flags,
@@ -8024,6 +8024,7 @@ ip4_address_add (NMPlatform *platform,
 	                             nm_utils_ip4_address_is_link_local (addr) ? RT_SCOPE_LINK : RT_SCOPE_UNIVERSE,
 	                             lifetime,
 	                             preferred,
+	                             broadcast_address,
 	                             label);
 
 	nmp_object_stackinit_id_ip4_address (&obj_id, ifindex, addr, plen, peer_addr);
@@ -8054,6 +8055,7 @@ ip6_address_add (NMPlatform *platform,
 	                             RT_SCOPE_UNIVERSE,
 	                             lifetime,
 	                             preferred,
+	                             0,
 	                             NULL);
 
 	nmp_object_stackinit_id_ip6_address (&obj_id, ifindex, &addr);
@@ -8077,6 +8079,7 @@ ip4_address_delete (NMPlatform *platform, int ifindex, in_addr_t addr, guint8 pl
 	                             RT_SCOPE_NOWHERE,
 	                             NM_PLATFORM_LIFETIME_PERMANENT,
 	                             NM_PLATFORM_LIFETIME_PERMANENT,
+	                             0,
 	                             NULL);
 	if (!nlmsg)
 		g_return_val_if_reached (FALSE);
@@ -8102,6 +8105,7 @@ ip6_address_delete (NMPlatform *platform, int ifindex, struct in6_addr addr, gui
 	                             RT_SCOPE_NOWHERE,
 	                             NM_PLATFORM_LIFETIME_PERMANENT,
 	                             NM_PLATFORM_LIFETIME_PERMANENT,
+	                             0,
 	                             NULL);
 	if (!nlmsg)
 		g_return_val_if_reached (FALSE);