ceph: fix comment over ceph_drop_caps_for_unlink

It's not clear what AUTH_RDCACHE means in this context, and we're
clearly just dropping LINK caps here.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>