libceph: factor out __ceph_x_decrypt()

Will be used for decrypting the server challenge which is only preceded
by ceph_x_encrypt_header.

Drop struct_v check to allow for extending ceph_x_encrypt_header in the
future.

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