BUG/MINOR: resolvers: Use resolver's lock in resolv_srvrq_expire_task()

The commit dcac41806 ("BUG/MEDIUM: resolvers: Add a task on servers to check
SRV resolution status") introduced a type. In resolv_srvrq_expire_task()
function, the resolver's lock must be used instead of the resolver itself.

This patch must be backported with the patch above (at least as far as 2.2).
diff --git a/src/resolvers.c b/src/resolvers.c
index a068137..c05d2c5 100644
--- a/src/resolvers.c
+++ b/src/resolvers.c
@@ -609,9 +609,9 @@
 	if (!tick_is_expired(t->expire, now_ms))
 		goto end;
 
-	HA_SPIN_LOCK(DNS_LOCK, &srv->srvrq->resolvers);
+	HA_SPIN_LOCK(DNS_LOCK, &srv->srvrq->resolvers->lock);
 	resolv_srvrq_cleanup_srv(srv);
-	HA_SPIN_UNLOCK(DNS_LOCK, &srv->srvrq->resolvers);
+	HA_SPIN_UNLOCK(DNS_LOCK, &srv->srvrq->resolvers->lock);
 
   end:
 	return t;