fstests: btrfs/301: handle auto-removed qgroups

fstests: btrfs/301: handle auto-removed qgroups

There are always attempts to auto-remove empty qgroups after dropping a
subvolume.

For squota mode, not all qgroups can or should be dropped, as there are
common cases where the dropped subvolume are still referred by other
snapshots.
In that case, the numbers can only be freed when the last referencer
got dropped.

The latest kernel attempt would only try to drop empty qgroups for
squota mode.
But even with such safe change, the test case still needs to handle
auto-removed qgroups, by explicitly echoing "0", or later calculation
would break bash grammar.

This patch would add extra handling for such removed qgroups, to be
future proof for qgroup auto-removal behavior change.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>