core: initialize l3cd dns-priority for ppp and wwan

For devices that configure IP by themselves (by returning
"->ready_for_ip_config() = TRUE" and implementing
->act_stage3_ip_config()), we skip manual configuration. Currently,
manual configuration is the only one that sets flag HAS_DNS_PRIORITY
into the resulting l3cd.

So, the merged l3cd for such devices misses a dns-priority and is
ignored by the DNS manager.

Explicitly initialize the priority to 0; in this way, the default
value for the device will be set in the final l3cd during the merge.

Fixes: 58287cbcc0 ('core: rework IP configuration in NetworkManager using layer 3 configuration')

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/issues/931

src/core/devices/wwan/nm-modem-broadband.c

@@ -1032,6 +1032,7 @@ stage3_ip_config_start(NMModem *modem, int addr_family, NMModemIPMethod ip_metho
         l3cd = nm_l3_config_data_new(nm_platform_get_multi_idx(NM_PLATFORM_GET),
                                      ifindex,
                                      NM_IP_CONFIG_SOURCE_WWAN);
+        nm_l3_config_data_set_dns_priority(l3cd, AF_INET, 0);
 
         address = (NMPlatformIP4Address){
             .address      = address_network,
@@ -1118,6 +1119,7 @@ stage3_ip_config_start(NMModem *modem, int addr_family, NMModemIPMethod ip_metho
         l3cd = nm_l3_config_data_new(nm_platform_get_multi_idx(NM_PLATFORM_GET),
                                      ifindex,
                                      NM_IP_CONFIG_SOURCE_WWAN);
+        nm_l3_config_data_set_dns_priority(l3cd, AF_INET6, 0);
 
         do_auto = TRUE;
 

src/core/ppp/nm-ppp-manager.c

@@ -545,6 +545,7 @@ impl_ppp_manager_set_ip4_config(NMDBusObject                      *obj,
                                  NM_IP_CONFIG_SOURCE_PPP);
 
     nm_l3_config_data_set_mtu(l3cd, mtu);
+    nm_l3_config_data_set_dns_priority(l3cd, AF_INET, 0);
 
     address = (NMPlatformIP4Address){
         .plen = 32,