Disable user space reference tracking

[behlendorf]

Motivation and Context

The memory and cpu cost of reference count tracking with the current
implementation is significant. For this reason it has always been
disabled by default for the kmods. Apply this same default to user
space so ztest doesn't always incur this performance penalty.

Our intention is to reenable this by default for ztest once the code
has been optimized. Since we expect to at some point provide a FUSE
implementation we wouldn't want this enabled by default for libzpool.

Note: There's some reason to believe this may eliminate / reduce a
class of ztest failures observed by the CI but not in local testing.

Description

By default disable user space reference tracking.

How Has This Been Tested?

Locally compiled and ran zloop.sh.

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-io]

Codecov Report

Merging #10189 into master will decrease coverage by 0.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10189      +/-   ##
==========================================
- Coverage   79.40%   79.33%   -0.07%     
==========================================
  Files         385      385              
  Lines      122603   122603              
==========================================
- Hits        97350    97271      -79     
- Misses      25253    25332      +79     

Flag	Coverage Δ
#kernel	79.75% <ø> (+0.13%)	⬆️
#user	66.11% <ø> (-0.44%)	⬇️

Impacted Files	Coverage Δ
module/zfs/refcount.c	84.72% <ø> (-6.95%)	⬇️
module/zfs/vdev_indirect.c	73.50% <0.00%> (-10.67%)	⬇️
cmd/zvol_id/zvol_id_main.c	76.31% <0.00%> (-5.27%)	⬇️
module/lua/lmem.c	83.33% <0.00%> (-4.17%)	⬇️
module/icp/api/kcf_mac.c	37.71% <0.00%> (-2.29%)	⬇️
module/zfs/zil.c	91.17% <0.00%> (-2.19%)	⬇️
module/zfs/arc.c	82.16% <0.00%> (-1.96%)	⬇️
module/zfs/btree.c	81.85% <0.00%> (-1.64%)	⬇️
module/zfs/metaslab.c	94.33% <0.00%> (-1.44%)	⬇️
... and 50 more

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