ceph: fix session check on mds reply

Fix a broken check that a reply came back from the same MDS we sent the
request to.  I don't think a case that actually triggers this would ever
come up in practice, but it's clearly wrong and easy to fix.

Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Sage Weil <sage@newdream.net>