There is a potential lock-order inversion between a worker thread

and the ban-lurker and there is nothing we can do about it:  They
come from opposite ends of the world.

Resolve this by using a TryLock in the ban-lurker and abandon the
attempt if we fail to get the lock.

Fixes:	#796


git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5432 d4fa192b-c00b-0410-8231-f00ffab90ce4

bin/varnishd/cache_hash.c

@@ -661,7 +661,10 @@ HSH_FindBan(struct sess *sp, struct objcore **oc)
 	CHECK_OBJ_NOTNULL(oc1, OBJCORE_MAGIC);
 	oh = oc1->objhead;
 	CHECK_OBJ_NOTNULL(oh, OBJHEAD_MAGIC);
-	Lck_Lock(&oh->mtx);
+	if (Lck_Trylock(&oh->mtx)) {
+		*oc = NULL;
+		return;
+	}
 	VTAILQ_FOREACH(oc2, &oh->objcs, list)
 		if (oc1 == oc2)
 			break;