Btrfs-progs: restructure list_subvolumes

Btrfs-progs: restructure list_subvolumes

The current code of list_subvols() has very bad scalability, if we want to
add new filter conditions or new sort methods, we have to modify lots of code.

Beside that, the most code of list_snapshots() is similar to list_subvols(),

So I restructure list_subvols(), and split the subvolume filter function,
the subvolume sort function and the output function from list_subvols().
In order to implement it, we defined some importtant structures:
struct btrfs_list_filter {
	btrfs_list_filter_func filter_func;
	void *data;
};

struct btrfs_list_comparer {
	btrfs_list_comp_func comp_func;
	int is_descending;
};

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

If we want to add a new filter condition, we can choose a suitable filter
function, or implement a new filter function[1], and add it into a set of
the filters, and then pass the filter set into list_subvols(). We also can
mix several filters (just add those filters into the set, and pass the set
into list_subvols()) if the users specify two or more filter conditions.

The subvolume sort function is similar to the subvolume filter function. The
differentiation is the order of comparers in the array which is passed into
list_subvols() show us the priority of the sort methods.

The output function is different with the above two functions, we define a
array 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.

After appling this patch, we needn't implement a independent list_snapshots()
function, just pass a filter function which is used to identify the snapshot
into list_subvols().

[1]: If we implement new filter functions or compare functions, we must add
them into the array all_filter_funcs or the array all_comp_funcs, and modify
the relative enum variants(btrfs_list_filter_enum, btrfs_list_comp_enum).

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>