btrfs-progs: qgroup: show 'none' when we did not limit it on this qgroup

btrfs-progs: qgroup: show 'none' when we did not limit it on this qgroup

There are two understanding of the '0' value in btrfs qgroup show.
(1) is no-limitation on this qgroup. (2) is the max-limitation is 0.

This patch make it showing in different way.

(1) max-limitation for 0 is still showing '0'.
(2) no-limitation will show 'none'.

qgroupid         rfer         excl     max_rfer     max_excl parent
--------         ----         ----     --------     -------- ------
0/5           2.19GiB      2.19GiB         none         none ---
0/257       100.02MiB    100.02MiB         none         none ---

Signed-off-by: Dongsheng Yang <yangds.fnst@cn.fujitsu.com>
Tested-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>