libceph: implement CEPHX_V2 calculation mode

Derive the signature from the entire buffer (both AES cipher blocks)
instead of using just the first half of the first block, leaving out
data_crc entirely.

This addresses CVE-2018-1129.

Link: http://tracker.ceph.com/issues/24837


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