btrfs: qgroup: Add function qgroup_update_refcnt().

This function is used to update refcnt for qgroups.
And is one of the two core functions used in the new qgroup implement.

This is based on the old update_old/new_refcnt, but provides a unified
logic and behavior.

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