Btrfs-progs: enhance btrfs qgroup show to sort qgroups

Btrfs-progs: enhance btrfs qgroup show to sort qgroups

You might want to list qgroups in order of some items, such as 'qgroupid', 'rfer'
and so on, you can use '--sort'. Now you can sort the qgroups by 'qgroupid',
'rfer','excl','max_rfer' and 'max_excl'.

For example:
	If you want to list qgroups in order of 'qgroupid'.
You can use the option like that:

	btrfs qgroup show --sort=+/-qgroupid <path>
Here, '+' means the result is sorted by ascending order. '-' is by descending
order. If you don't specify either '+' nor '-', the result is sorted by
default - ascending order.

If you want to combine sort items, you do it like that:
	btrfs qgroup show  --sort=-qgroupid,+rfer,max_rfer,excl <path>

Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>