Btrfs-progs: restructure show_qgroups

Btrfs-progs: restructure show_qgroups

The current show_qgroups() just shows a little information, and it is hard to
add some functions which the users need in the future, so i restructure it, make
it easy to add new functions.

In order to improve the scalability of show_qgroups(), i add some important
structures:

	struct qgroup_lookup {
		struct rb_root root;
	}
 	/*
	*store qgroup's information
	*/
	struct btrfs_qgroup {
		struct rb_node  rb_node;
		u64 qgroupid;

		u64 generation;
		u64 rfer;
		u64 rfer_cmpr;
		u64 excl_cmpr;

		u64 flags;
		u64 max_rfer;
		u64 max_excl;
		u64 rsv_rfer;
		u64 rsv_excl;

		struct list_head qgroups;
		struct list_head members;
	}
	/*
	*glue structure to represent the relations
	*between qgroups
	*/
	struct btrfs_qgroup_list {
		struct list_head next_qgroups;
		struct list_head next_member;
		struct btrfs_qgroup *qgroup;
		struct btrfs_qgroup *member;
	}
The above 3 structures are used to manage all the information
of qgroups.

	struct {
		char *name;
		char *column_name;
		int need_print;
	} btrfs_qgroup_columns[]

We define a arrary to manage all the columns that can be
outputed, and use a member variant(->need_print) to control
the output of the relative column. Some columns are outputed
by default. But we can change it according to the requirement
of the users.

For example:
	if outputing max referenced size of qgroup is needed,the function
'btrfs_qgroup_setup_column()' will be called, and the parameter 'BTRFS_QGROUP_MAX_RFER'
(extend in the future) will be passsed to the function. After the function is done,
when showing qgroups, max referenced size of qgroup will be output.

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>