ceph: properly queue cap snap for newly created snap realm

commit 3ae0bebc "ceph: queue cap snap only when snap realm's
context changes" introduced a regression: we may not call
queue_realm_cap_snaps() for newly created snap realm. This
regression allows unflushed snapshot data to be overwritten.

Link: http://tracker.ceph.com/issues/21483
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/snap.c

@@ -374,12 +374,10 @@ static int build_snap_context(struct ceph_snap_realm *realm,
 	     realm->ino, realm, snapc, snapc->seq,
 	     (unsigned int) snapc->num_snaps);
 
-	if (realm->cached_context) {
-		ceph_put_snap_context(realm->cached_context);
-		/* queue realm for cap_snap creation */
-		list_add_tail(&realm->dirty_item, dirty_realms);
-	}
+	ceph_put_snap_context(realm->cached_context);
 	realm->cached_context = snapc;
+	/* queue realm for cap_snap creation */
+	list_add_tail(&realm->dirty_item, dirty_realms);
 	return 0;
 
 fail: