xfstests: Speed up test 042

xfstests: Speed up test 042

test 042 generates a worst-case fragmented filesystem and uses it to
test xfs_fsr. It uses small 4k files to generate the hole-space-hole
pattern that fragments free space badly. It is much faster to
generate the same pattern by creating a single large file and
punching holes in it.  Also, instead of writing large files to
create unfragmented space, just use preallocation so we don't have
to write the data to disk.

These changes reduce the runtime of the test on a single SATA drive
from 106s to 27s.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Alex Elder <aelder@sgi.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>