libceph: store ceph_auth_handshake pointer in ceph_connection

We already copy authorizer_reply_buf and authorizer_reply_buf_len into
ceph_connection.  Factoring out __prepare_write_connect() requires two
more: authorizer_buf and authorizer_buf_len.  Store the pointer to the
handshake in con->auth rather than piling on.

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