libceph: store ceph_auth_handshake pointer in ceph_connection

commit 262614c4294d33b1f19e0d18c0091d9c329b544a upstream.

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>
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>