generic/413, xfs/260: Improve format operation for PMD fault testing

generic/413, xfs/260: Improve format operation for PMD fault testing

1) Simple code and fix the wrong value of stripe_width by _scratch_mkfs_geom().
2) Get hugepage size by _get_hugepagesize() and replace fixed 2M with
   hugepage size because hugepage size/PMD_SIZE is not 2M on some
   arches(e.g. hugepage size/PMD_SIZE is 512M on arm64).
3) For debugging, redirect the output of mkfs to $seqres.full.

Signed-off-by: Xiao Yang <yangx.jy@cn.fujitsu.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>