overlay/005: only run for xfs underlying fs

overlay/005: only run for xfs underlying fs

When we runs overlay/005 on a system without xfs module, it always
fails as "unknown filesystem type xfs", due to this case require xfs
to be the underlying fs explicitly:

  $MKFS_XFS_PROG -f -n ftype=1 $upper_loop_dev >>$seqres.full 2>&1

So notrun this case if the underlying fs isn't 'xfs'.

Reported-by: Philip Daly <pdaly@redhat.com>
Acked-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>