libceph: add support for CEPH_CRYPTO_AES256KRB5

libceph: add support for CEPH_CRYPTO_AES256KRB5

This is based on AES256-CTS-HMAC384-192 crypto algorithm per RFC 8009
(i.e. Kerberos 5, hence the name) with custom-defined key usage numbers.
The implementation allows a given key to have/be linked to between one
and three usage numbers.

The existing CEPH_CRYPTO_AES remains in place and unchanged.  The
usage_slot parameter that needed to be added to ceph_crypt() and its
wrappers is simply ignored there.

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