mds: handle discovers that race with refragmenting

mds: handle discovers that race with refragmenting

Consider:

 - send discover on frag X
 - X refragments
   - we take the waiter and rediscover on frag Y
 - we get the reply for the X discover

The auth mds will correctly delay sending the reply until the refragment
completes and it unfreezes, but the reply was getting the original frag_t,
not the new one.

Signed-off-by: Sage Weil <sage@newdream.net>