ceph: add status debugfs file

This will help list some useful client side info, like the client
entity address/name and blocklisted status, etc.

URL: https://tracker.ceph.com/issues/48057


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>