xfstests: add --largefs check option

xfstests: add --largefs check option

Make it easier to check large filesystems quickly by adding a
--large-fs option to check to turn on shortcuts for large scratch
device filesystem testing.

Also, reject invalid command line options with a usage message.

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>