BUG/MINOR: dns: Don't try to get the server lock if it's already held.

dns_link_resolution() can be called with the server lock already held, so
don't attempt to lock it again in that case.
diff --git a/src/server.c b/src/server.c
index ed78ca5..adc9fd4 100644
--- a/src/server.c
+++ b/src/server.c
@@ -3818,7 +3818,7 @@
 	if (!srv->hostname || !srv->hostname_dn)
 		goto err;
 
-	if (dns_link_resolution(srv, OBJ_TYPE_SERVER) == -1)
+	if (dns_link_resolution(srv, OBJ_TYPE_SERVER, 1) == -1)
 		goto err;
 
   end: