common/xfs: fix unexpected failure on real 4k sector device

common/xfs: fix unexpected failure on real 4k sector device

_xfs_mkfs_validation_check() run "$cmd -s size=2s" to be sure if
xfsprogs support stricter input checks. But there's an unexpected
failure on 4k sector device:

  # blockdev --getbsz --getpbsz --getss $dev
  4096
  4096
  4096
  # mkfs.xfs -f -N -d file,name=$tmpfile,size=1g -s size=2s
  illegal sector size 1024; hw sector is 4096
  ....

So change the sector size from '2s' to '8s' to keep away this
failure.

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