xfstests: enable large fs testing on ext4

xfstests: enable large fs testing on ext4

Now that setting up large filesystem testing on sparse loopback
devices uses a generic method for filling the filesystem, extent
support to ext4 filesystems.

ext4 is slightly more complex to fill as it does not support files
larger than 16TB. Hence a slightly more complex method of using
multiple smaller files to fill the space is necessary.

WARNING: be prepared for ext4 to take forever to prepare large
filesystems as allocation of large amounts of space (especially as
it approaches ENOSPC) is can take minutes rather than milliseconds

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>