FreeBSD: add support for lockless lookup

[mattmacy]

Authored-by: mjg mjg@FreeBSD.org
Signed-off-by: Matt Macy mmacy@FreeBSD.org

Motivation and Context

Description

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]

Codecov Report

Merging #10657 into master will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10657      +/-   ##
==========================================
+ Coverage   79.55%   79.67%   +0.12%     
==========================================
  Files         394      394              
  Lines      124631   124631              
==========================================
+ Hits        99146    99301     +155     
+ Misses      25485    25330     -155     

Flag	Coverage Δ
#kernel	80.14% <ø> (+0.01%)	⬆️
#user	65.86% <ø> (+0.94%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
module/zfs/vdev_indirect.c	73.50% <0.00%> (-11.50%)	⬇️
module/os/linux/spl/spl-kmem-cache.c	78.13% <0.00%> (-10.74%)	⬇️
module/zfs/bpobj.c	86.86% <0.00%> (-4.29%)	⬇️
lib/libzfs/libzfs_changelist.c	85.01% <0.00%> (-1.13%)	⬇️
module/zcommon/zfs_uio.c	87.75% <0.00%> (-1.03%)	⬇️
module/zfs/spa_log_spacemap.c	93.40% <0.00%> (-0.86%)	⬇️
module/zfs/dmu_traverse.c	95.68% <0.00%> (-0.67%)	⬇️
cmd/zed/agents/zfs_mod.c	77.55% <0.00%> (-0.67%)	⬇️
module/zfs/zfs_fm.c	84.39% <0.00%> (-0.58%)	⬇️
cmd/zdb/zdb.c	81.70% <0.00%> (-0.52%)	⬇️
... and 54 more

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