libceph: generalize ceph_x_encrypt_offset() and ceph_x_encrypt_buflen()

libceph: generalize ceph_x_encrypt_offset() and ceph_x_encrypt_buflen()

- introduce the notion of a data offset for ceph_x_encrypt_offset()
  to allow for e.g. confounder to be prepended before the encryption
  header in the future.  For CEPH_CRYPTO_AES, the data offset is 0
  (i.e. nothing is prepended).

- adjust ceph_x_encrypt_buflen() accordingly and make it account for
  PKCS#7 padding that is used by CEPH_CRYPTO_AES precisely instead of
  just always adding 16.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>