l3cfg: make nm_l3_config_data_cmp_full() thread-safe

Let's not have unexpected, non-thread-safe functions somewhere deep down.

NML3ConfigData -- as a data structure -- is not thread-safe, nor aims it to
be. However, our code(!) should be thread-safe. That means, it should be
possible to call our code on separate data from multiple threads.
Violating that is a code smell and a foot gun.

This basically means that code should not access global data (unless
thread-local) or that the access to global-data needs to be
synchronized.

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/1806

src/core/nm-l3-config-data.c

@@ -2231,14 +2231,23 @@ static const NML3ConfigData *
 get_empty_l3cd(void)
 {
     static NML3ConfigData *empty_l3cd;
+    NML3ConfigData        *l3cd;
 
-    if (!empty_l3cd) {
-        empty_l3cd =
-            nm_l3_config_data_new(nm_dedup_multi_index_new(), 1, NM_IP_CONFIG_SOURCE_UNKNOWN);
-        empty_l3cd->ifindex = 0;
+again:
+    l3cd = g_atomic_pointer_get(&empty_l3cd);
+    if (G_UNLIKELY(!l3cd)) {
+        l3cd = nm_l3_config_data_new(nm_dedup_multi_index_new(), 1, NM_IP_CONFIG_SOURCE_UNKNOWN);
+        l3cd->ifindex = 0;
+
+        nm_l3_config_data_seal(l3cd);
+
+        if (!g_atomic_pointer_compare_and_exchange(&empty_l3cd, NULL, l3cd)) {
+            nm_l3_config_data_unref(l3cd);
+            goto again;
+        }
     }
 
-    return empty_l3cd;
+    return l3cd;
 }
 
 int