btrfs: qgroup: move WARN_ON() to the correct location.

btrfs: qgroup: move WARN_ON() to the correct location.

In function qgroup_excl_accounting(), we need to WARN when
qg->excl is less than what we want to free, same to child
and parents. But currently, for parent qgroup, the WARN_ON()
is located after freeing qg->excl. It will WARN out even we
free it normally.

This patch move this WARN_ON() before freeing qg->excl.

Signed-off-by: Dongsheng Yang <[email protected]>
Reviewed-by: Satoru Takeuchi <[email protected]>
Signed-off-by: Chris Mason <[email protected]>