doc: ceph: update userspace command to get CephFS metadata

According to ceph documentation [1], "getfattr -d /some/dir" no longer
displays the list of all extended attributes. Both CephFS kernel and
FUSE clients hide this information.

To retrieve the information you have to specify the particular attribute
name e.g. "getfattr -n ceph.dir.rbytes /some/dir".

[1] https://docs.ceph.com/en/latest/cephfs/quota/



Signed-off-by: Artem Ikonnikov <artem@datacrunch.io>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>