Btrfs-progs: list all qgroups impact given path(exclude ancestral qgroups)

Btrfs-progs: list all qgroups impact given path(exclude ancestral qgroups)

This patch introduces '-f' option which can help you filter the qgroups
by the path name, you may use it like:

	btrfs qgroup show -f <path>
For example:

                         qgroupid(2/0)
                          /     \
                         /       \
                        qgroupid(1/0)
                        /         \
                       /           \
                      /             \
                  qgroupid(0/1)   qgroupid(0/2)
                  sub1              sub2
                  /  \
                 /    \
		dir1  file1

If we use the command:
	btrfs qgroup show -f sub1/dir1
The result will output
	0/1	--	--

'-f' option helps you list all qgroups impact given path.
(exclude ancestral qgroups)

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>