libceph: no need to drop con->mutex for ->get_authorizer()

->get_authorizer(), ->verify_authorizer_reply(), ->sign_message() and
->check_message_signature() shouldn't be doing anything with or on the
connection (like closing it or sending messages).

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>