libceph: prevent potential out-of-bounds writes in handle_auth_session_key()

The len field originates from untrusted network packets. Boundary
checks have been added to prevent potential out-of-bounds writes when
decrypting the connection secret or processing service tickets.

[ idryomov: changelog ]

Cc: stable@vger.kernel.org
Signed-off-by: ziming zhang <ezrakiez@gmail.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>