Call __validate_auth() under monc->mutex, and use helper for
initial hello so that the pending_auth flag is set.  This fixes
possible races in which we have an authentication request (hello
or otherwise) pending and send another one.  In particular, with
auth_none, we _never_ want to call ceph_build_auth() from
__validate_auth(), since the ->build_request() method is NULL.
Signed-off-by: Sage Weil <sage@newdream.net>