btrfs: qgroup: Cleanup old inaccurate facilities

Cleanup the old facilities which use old btrfs_qgroup_reserve() function
call, replace them with the newer version, and remove the "__" prefix in
them.

Also, make btrfs_qgroup_reserve/free() functions private, as they are
now only used inside qgroup codes.

Now, the whole btrfs qgroup is swithed to use the new reserve facilities.

Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>