libceph: check authorizer reply/challenge length before reading

commit 130f52f2b203aa0aec179341916ffb2e905f3afd upstream.

Avoid scribbling over memory if the received reply/challenge is larger
than the buffer supplied with the authorizer.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Sage Weil <sage@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>