generic: test online label ioctl

generic: test online label ioctl

This tests the online label ioctl that btrfs has, which has been
recently proposed for XFS.

To run, it requires an updated xfs_io with the label command and a
filesystem that supports it

A slight change here to _require_xfs_io_command as well, so that
tests which simply fail with "Inappropriate ioctl" can be caught in
the common case.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>