libceph: rename and align ceph_x_authorizer::reply_buf

It's going to be used as a temporary buffer for in-place en/decryption
with ceph_crypt() instead of on-stack buffers, so rename to enc_buf.
Ensure alignment to avoid GFP_ATOMIC allocations in the crypto stack.

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