libceph: wrap auth methods in a mutex

The auth code is called from a variety of contexts, include the mon_client
(protected by the monc's mutex) and the messenger callbacks (currently
protected by nothing).  Avoid chaos by protecting all auth state with a
mutex.  Nothing is blocking, so this should be simple and lightweight.

Signed-off-by: Sage Weil <sage@inktank.com>
libceph: verify authorizer reply

The 'cephx' auth protocol provides mutual authenticate for client and
server.  However, as the client, we were not verifying that the server
auth reply was in fact authentic.  Although the infrastructure to do so was
all in place, we neglected to actually call the function to do it.  Fix!

This resolves http://tracker.ceph.com/issues/2429.

Reported-by: Alex Elder <elder@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>