Make spa_stats.c tunables visible on FreeBSD

Use ZFS_MODULE_PARAM for cross-platform tunables in spa_stats.c, and
add update tunables.cfg in tests for the newly supported ones.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10858