btrfs: detect regular qgroup for older kernels correctly

btrfs: detect regular qgroup for older kernels correctly

[BUG]
When running an older (vendoer v6.4) kernel, some qgroup test cases
would be skipped:

  btrfs/017 1s ... [not run] not running normal qgroups

[CAUSE]
With the introduce of simple quota mode, there is a new sysfs interface,
/sys/fs/btrfs/<uuid>/qgroups/mode to indicate the currently running
qgroup modes.

And _qgroup_mode() from `common/btrfs` is using that new interface to
detect the mode.

Unfortuantely for older kernels without simple quota support,
_qgroup_mode() would return "disabled" directly, causing those test case
to be skipped.

[FIX]
Fallback to regular qgroup if that sysfs interface is not accessible, as
qgroup is introduced from the very beginning of btrfs, thus the regular
qgroup is always supported.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>