btrfs-progs: qgroup: allow user to clear some limitation on qgroup.

btrfs-progs: qgroup: allow user to clear some limitation on qgroup.

Currently, we can not clear a limitation on a qgroup. Although
there is a 'none' choice provided to user to do it, it does not
work well.

It does not set the flag which user want to clear, then kernel
will never know what the user want to do at all.

*Without this commit*
 # ./btrfs qgroup show -re /mnt
qgroupid         rfer         excl     max_rfer     max_excl
--------         ----         ----     --------     --------
0/5           2.19GiB      2.19GiB      5.00GiB         none
0/257       100.02MiB    100.02MiB         none         none
 # ./btrfs qgroup limit none /mnt
 # ./btrfs qgroup show -re /mnt
qgroupid         rfer         excl     max_rfer     max_excl
--------         ----         ----     --------     --------
0/5           2.19GiB      2.19GiB      5.00GiB         none
0/257       100.02MiB    100.02MiB         none         none

This patch will set the flag user want to clear and pass a
size=-1 to kernel. Then kernel will clear it correctly.

*With this commit*
 # ./btrfs qgroup show -re /mnt
qgroupid         rfer         excl     max_rfer     max_excl
--------         ----         ----     --------     --------
0/5           2.19GiB      2.19GiB      5.00GiB         none
0/257       100.02MiB    100.02MiB         none         none
 # ./btrfs qgroup limit none /mnt
 # ./btrfs qgroup show -re /mnt
qgroupid         rfer         excl     max_rfer     max_excl
--------         ----         ----     --------     --------
0/5           2.19GiB      2.19GiB         none         none
0/257       100.02MiB    100.02MiB         none         none

Reported-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
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>