ZTS: Reduce file size in redacted_panic to 1GB

This test takes 3 minutes on RELEASE FreeBSD bots, but on CURRENT,
probably due to debugging it has in kernel, it does not complete
within 10 minutes, ending up killed. As I see all the redacting
here happens within the first ~128MB of the file, so I hope it
won't matter if there is 1GB of data instead of 2GB.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by:Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #11141