libceph: align session_key and con_secret to 16 bytes

crypto_shash_setkey() and crypto_aead_setkey() will do a (small)
GFP_ATOMIC allocation to align the key if it isn't suitably aligned.
It's not a big deal, but at the same time easy to avoid.

The actual alignment requirement is dynamic, queryable with
crypto_shash_alignmask() and crypto_aead_alignmask(), but shouldn't
be stricter than 16 bytes for our algorithms.

Fixes: cd1a677c ("libceph, ceph: implement msgr2.1 protocol (crc and secure modes)")
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>