arc: Periodically update ARC kstats

[ghost]

Motivation and Context

FreeBSD needs arc_adjust_zthr to run periodically for kstats to be
updated. A comment in the code suggests this may have been the
original intent in illumos as well:

zfs/module/zfs/arc.c

Lines 4697 to 4700 in c946d5a

	* anon, mru, mru_ghost, mfu, and mfu_ghost lists. Even
	* with this change, the data might be up to 1 second
	* out of date(the arc_adjust_zthr has a maximum sleep
	* time of 1 second); but that should suffice. The

Description

Create the thread with a 1 second timer.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the ZFS on Linux code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov-commenter]

Codecov Report

Merging #10371 into master will increase coverage by 0.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #10371      +/-   ##
==========================================
+ Coverage   79.52%   79.75%   +0.23%     
==========================================
  Files         391      391              
  Lines      123590   123590              
==========================================
+ Hits        98288    98575     +287     
+ Misses      25302    25015     -287     

Flag	Coverage Δ
#kernel	80.24% <100.00%> (+0.23%)	⬆️
#user	66.54% <100.00%> (+0.60%)	⬆️

Impacted Files	Coverage Δ
module/zfs/arc.c	89.60% <100.00%> (+0.76%)	⬆️
module/zfs/dsl_scan.c	84.95% <0.00%> (-1.60%)	⬇️
module/zfs/vdev_raidz.c	92.59% <0.00%> (-0.66%)	⬇️
module/zfs/vdev_indirect_mapping.c	98.55% <0.00%> (-0.49%)	⬇️
module/zfs/dnode.c	94.59% <0.00%> (-0.37%)	⬇️
lib/libefi/rdwr_efi.c	60.37% <0.00%> (-0.32%)	⬇️
module/zfs/metaslab.c	95.87% <0.00%> (-0.28%)	⬇️
module/zfs/dmu.c	86.42% <0.00%> (-0.12%)	⬇️
module/os/linux/zfs/zfs_znode.c	85.39% <0.00%> (-0.12%)	⬇️
lib/libzfs/libzfs_pool.c	73.41% <0.00%> (-0.06%)	⬇️
... and 46 more

---

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[ghost]

I thought this got committed, weird

[mattmacy]

This was in the initial port and was only reverted to minimize the diff when merging.