zfs_debug: Restore log size limit for userspace

For some reason it was dropped when split from kernel, that makes
raidz_test to accumulate in RAM up to 100GB of logs we don't need.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Rob Norris <robn@despairlabs.com>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16492
Closes #16566
Closes #16664