btrfs: slightly loosen the requirement for qgroup removal

[BUG]
Currently if one is utilizing "qgroups/drop_subtree_threshold" sysfs,
and a snapshot with level higher than that value is dropped, we will
not be able to delete the qgroup until next qgroup rescan:

  uuid=ffffffff-eeee-dddd-cccc-000000000000

  wipefs -fa $dev
  mkfs.btrfs -f $dev -O quota -s 4k -n 4k -U $uuid
  mount $dev $mnt

  btrfs subvolume create $mnt/subv1/
  for (( i = 0; i < 1024; i++ )); do
  	xfs_io -f -c "pwrite 0 2k" $mnt/subv1/file_$i > /dev/null
  done
  sync
  btrfs subvolume snapshot $mnt/subv1 $mnt/snapshot
  btrfs quota enable $mnt
  btrfs quota rescan -w $mnt
  sync
  echo 1 > /sys/fs/btrfs/$uuid/qgroups/drop_subtree_threshold
  btrfs subvolume delete $mnt/snapshot
  btrfs subvolume sync $mnt
  btrfs qgroup show -prce --sync $mnt
  btrfs qgroup destroy 0/257 $mnt
  umount $mnt

The final qgroup removal would fail with the following error:

  ERROR: unable to destroy quota group: Device or resource busy

[CAUSE]
The above script would generate a subvolume of level 2, then snapshot
it, enable qgroup, set the drop_subtree_threshold, then drop the
snapshot.

Since the subvolume drop would meet the threshold, qgroup would be
marked inconsistent and skip accounting to avoid hanging the system at
transaction commit.

But currently we do not allow a qgroup with any rfer/excl numbers to be
dropped, and this is not really compatible with the new
drop_subtree_threshold behavior.

[FIX]
Only require the strict zero rfer/excl/rfer_cmpr/excl_cmpr for squota
mode.  This is due to the fact that squota can never go inconsistent,
and it can have dropped subvolume but with non-zero qgroup numbers for
future accounting.

For full qgroup mode, we only check if there is a subvolume for it.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>