xfstests: randholes: a few final cleanups

xfstests: randholes: a few final cleanups

Minor bits to wrap up this series.
- Bumped up the number of blocks read at once from 10 to 256
- Shorten remaining long lines
- Re-factor findblock()
- Get rid of global variable "nvalid"
- Add a few more comments
- Give all global symbols static scope
- Update the copyright message
- Various other minor formatting changes.

Signed-off-by: Alex Elder <aelder@sgi.com>
Acked-by: Dave Chinner <david@fromorbit.com>