ceph: fix auth cap handling logic in remove_session_caps_cb

The existing logic relies on ci->i_auth_cap being NULL, but if we end up
removing the auth cap early, then we'll do a lot of useless work and
lock-taking on the remaining caps. Ensure that we only do the auth cap
removal when we're _actually_ removing the auth cap.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>