platform: track ifa_flags for cached IPv4 addresses

It's useful to track the flags for IPv4 addresses too.

- we might want to sort IPv4 addresses according to whether they
  are permanent or dynamic.

- later we want to set IFA_F_NOPREFIXROUTE also for IPv4 addresses.
  While the ability to "set" a flag doesn't necessarily require that we
  also keep the flags present in NMPlatformIP4Address, it is more consistent.

src/platform/nm-linux-platform.c

@@ -1673,11 +1673,9 @@ _new_from_nl_addr (struct nlmsghdr *nlh, gboolean id_only)
 
 	obj->ip_address.source = NM_IP_CONFIG_SOURCE_KERNEL;
 
-	if (!is_v4) {
-		obj->ip6_address.n_ifa_flags = tb[IFA_FLAGS]
-		                               ? nla_get_u32 (tb[IFA_FLAGS])
-		                               : ifa->ifa_flags;
-	}
+	obj->ip_address.n_ifa_flags = tb[IFA_FLAGS]
+	                              ? nla_get_u32 (tb[IFA_FLAGS])
+	                              : ifa->ifa_flags;
 
 	if (is_v4) {
 		if (tb[IFA_LABEL]) {