xfstests: use preallocation for ag-wiper

xfstests: use preallocation for ag-wiper

To enable sane testing of large scale filesystems, the --large-fs
test option uses xfs_db magic to mark AGs full without doing any IO.
This leaves only a small amount of free space left in the filesystem
to stress the high AGs of the filesystem rather than the low AGs.

This method requires us to have special filesystem check options to
avoid free space checking in xfs_check, and we cannot current run
xfs_repair on such a filesystem at all. As it is, free space
checking on xfs_check does not scale, so we still need to avoid this
checking regardless of how we fill the filesystem.

We can achieve exactly the same fill behavior by preallocating a
single large file in the filesystem immediately after creating it.
This is a filesystem independent manner of filling the filesystem,
and allows us to do large filesystem testing on more than just XFS.

Further, this preallocation method effectively adds a new "very
large file" test. It also enables us to run an unmodified xfs_repair
or filesystem specific fsck program to check the filesystem for
sanity, so we can now do full sanity checking of such large
filesystems.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>