btrfs: check qgroup on extent de-reference

btrfs: check qgroup on extent de-reference

Test if qgroup can handle extent de-reference during reallocation.
"extent de-reference" means that reducing an extent's reference
count or freeing an extent.

Although current qgroup can handle it, we still need to prevent any
regression which may break current qgroup.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>