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

In the Linux kernel, the following vulnerability has been resolved: 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 ]