Do not try to expire objects that are still busy.

There is a short window of opportunity between HSH_Insert() and
HSH_Unbusy(), where the locks are released and the expiry thread can
try to expire an object that is still busy (for objects with very
short TTL). This patch makes the expiry thread check for busy objects
and wait if the head of the binheap is busy.

No test case as the window is too small.

Fixes: #1150

bin/varnishd/cache_expire.c

@@ -371,6 +371,13 @@ exp_timer(struct sess *sp, void *priv)
 			continue;
 		}
 
+		/* If the object is busy, we have to wait for it */
+		if (oc->flags & OC_F_BUSY) {
+			Lck_Unlock(&exp_mtx);
+			oc = NULL;
+			continue;
+		}
+
 		/*
 		 * It's time...
 		 * Technically we should drop the exp_mtx, get the lru->mtx