Fix zfs_xattr_owner_unlinked on FreeBSD and comment

[mattmacy]

Explain FreeBSD VFS' unfortunate idiosyncratic locking requirements.

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.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.

[codecov]

Codecov Report

Merging #9720 into master will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #9720    +/-   ##
========================================
+ Coverage      79%      79%   +<1%     
========================================
  Files         420      419     -1     
  Lines      123654   123612    -42     
========================================
+ Hits        98069    98242   +173     
+ Misses      25585    25370   -215

Flag	Coverage Δ
#kernel	80% <ø> (ø)	⬆️
#user	67% <ø> (+1%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ddb4e69...59dde84. Read the comment docs.