xfs/288: notrun if xfs_db write command fails

xfs/288: notrun if xfs_db write command fails

Old xfsprogs can't change attr hdr.count to 0 on v5 filesystems, two
reasons maybe cause this issue:

1) This commit has been merged: 89baf918(xfs_db: write values into
   dir/attr blocks and recalculate CRCs).
2) xfs_db write command doesn't support -d option.

That's not a real bug, so skip this test if xfs_db can't set attr
hdr.count to 0.

Signed-off-by: Zorro Lang <zlang@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>