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/include/proto/dns.h b/include/proto/dns.h
index c3e3846..3ad79c3 100644
--- a/include/proto/dns.h
+++ b/include/proto/dns.h
@@ -40,7 +40,7 @@
                              void **newip, short *newip_sin_family,
                              void *owner);
 
-int dns_link_resolution(void *requester, int requester_type);
+int dns_link_resolution(void *requester, int requester_type, int requester_locked);
 void dns_unlink_resolution(struct dns_requester *requester);
 void dns_trigger_resolution(struct dns_requester *requester);