cephx: sign messages using hmac_sha256

cephx: sign messages using hmac_sha256

if key type is newer than the original AES, calculate message
hash by using HMAC-SHA256.
We cannot use plain aes256k like we do with the aes key because
of the confounder. The other option would be to inject a
confounder, but that would weaken the cipher.

Signed-off-by: Yehuda Sadeh <ysadehwe@ibm.com>
(cherry picked from commit ba6bb55c7c977e9858e242e74d848273617c221b)