274: Many fixups

274: Many fixups

This changes quite a few things about 274 to make it more robust
and useful.

* More comments
* Use xfs_io for falloc (not all systems have /usr/bin/fallocate)
* use _require_xfs_io_falloc to be sure system & fs support preallocation
* Do not remove all of the files in $SCRATCH_MNT/ post-mkfs
* Do not remove all of the files in $SCRATCH_MNT/ on completion
  (this breaks e2fsck when lost+found/ goes missing)
* Don't cd into $SCRATCH_MNT
* Try harder to completely fill the fs
* Use a larger preallocated space, and write into all of it (hopefully
  avoid just accidentally succeeding by writing into fs reserved
  space that may be there)
* Save more output in $seq.full instead of /dev/null
* Fill preallocated space diabolically

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Reviewed-by: Jan Kara <jack@suse.cz>