libceph: adapt ceph_x_challenge_blob hashing and msgr1 message signing

libceph: adapt ceph_x_challenge_blob hashing and msgr1 message signing

The existing approach where ceph_x_challenge_blob is encrypted with the
client's secret key and then the digest derived from the ciphertext is
used for the test doesn't work with CEPH_CRYPTO_AES256KRB5 because the
confounder randomizes the ciphertext: the client and the server get two
different ciphertexts and therefore two different digests.

msgr1 signatures are affected the same way: a digest derived from the
ciphertext for the message's "sigblock" is what becomes a signature and
the two sides disagree on the expected value.

For CEPH_CRYPTO_AES256KRB5 (and potential future encryption schemes),
switch to HMAC-SHA256 function keyed in the same way as the existing
encryption.  For CEPH_CRYPTO_AES, everything is preserved as is.

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