btrfs: qgroup: Drop fs_info parameter from btrfs_qgroup_trace_extent

It can be fetched from the transaction handle. In addition, remove the
WARN_ON(trans == NULL) because it's not possible to hit this condition.

Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>