networkd: link - do not drop config for loopback device

Commit 5e5b137 (networkd: link - drop foreign config when configuring
link) introduced a regression where addresses (including 127.0.0.1) are
removed from loopback device.
Do not handle loopback device when removing foreign configs.

Signed-off-by: Christian Hesse <mail@eworm.de>

src/network/networkd-link.c

@@ -2040,9 +2040,13 @@ static int link_configure(Link *link) {
         assert(link->network);
         assert(link->state == LINK_STATE_PENDING);
 
-        r = link_drop_foreign_config(link);
-        if (r < 0)
-                return r;
+        /* Drop foreign config, but ignore loopback device.
+         * We do not want to remove loopback address. */
+        if (!(link->flags & IFF_LOOPBACK)) {
+                r = link_drop_foreign_config(link);
+                if (r < 0)
+                        return r;
+        }
 
         r = link_set_bridge_fdb(link);
         if (r < 0)