libceph: fix msgpool

There were several problems here:

 1- we weren't tagging allocations with the pool, so they were never
    returned to the pool.
 2- msgpool_put didn't add back to the mempool, even it were called.
 3- msgpool_release didn't clear the pool pointer, so it would have looped
    had #1 not been broken.

These may or may not have been responsible for #1136 or #1381 (BUG due to
non-empty mempool on umount).  I can't seem to trigger the crash now using
the method I was using before.

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