ceph: fix snap rebuild condition

We were rebuilding the snap context when it was not necessary
(i.e. when the realm seq hadn't changed _and_ the parent seq
was still older), which caused page snapc pointers to not match
the realm's snapc pointer (even though the snap context itself
was identical).  This confused begin_write and put it into an
endless loop.

The correct logic is: rebuild snapc if _my_ realm seq changed, or
if my parent realm's seq is newer than mine (and thus mine needs
to be rebuilt too).
Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/snap.c

@@ -314,9 +314,9 @@ static int build_snap_context(struct ceph_snap_realm *realm)
 	   because we rebuild_snap_realms() works _downward_ in
 	   hierarchy after each update.) */
 	if (realm->cached_context &&
-	    realm->cached_context->seq <= realm->seq &&
+	    realm->cached_context->seq == realm->seq &&
 	    (!parent ||
-	     realm->cached_context->seq <= parent->cached_context->seq)) {
+	     realm->cached_context->seq >= parent->cached_context->seq)) {
 		dout("build_snap_context %llx %p: %p seq %lld (%d snaps)"
 		     " (unchanged)\n",
 		     realm->ino, realm, realm->cached_context,