Avoid extra work updating ARC kstats and tunables

After e357046 it should not be necessary to periodically update ARC
kstats and tunables. Tunable updates are applied when modified, and
kstats are updated on demand.

Update kstats in arc_evict_cb_check() for ZFS_DEBUG builds only.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11237