Make the ban lurker skip busy objects

If the ban lurker finds a busy object, skip the object and defer the
checking to a later pass.

No test case because it is very timing sensitive (but see the trac bug
report for a VTC-file that mostly triggers it on the 3.0 branch only)

Fixes: #1162

bin/varnishd/cache_ban.c

@@ -837,6 +837,18 @@ ban_lurker_work(const struct sess *sp, unsigned pass)
 				TIM_sleep(params->ban_lurker_sleep);
 				continue;
 			}
+			/*
+			 * If the object is busy, we can't touch
+			 * it. Defer it to a later run.
+			 */
+			if (oc->flags & OC_F_BUSY) {
+				oc->flags |= pass;
+				VTAILQ_REMOVE(&b->objcore, oc, ban_list);
+				VTAILQ_INSERT_TAIL(&b->objcore, oc, ban_list);
+				Lck_Unlock(&oh->mtx);
+				Lck_Unlock(&ban_mtx);
+				continue;
+			}
 			/*
 			 * Grab a reference to the OC and we can let go of
 			 * the BAN mutex