zvol_os: Code cleanup

[ghost]

Motivation and Context

Various issues were found while investigating -o volmode=xxx being ignored on FreeBSD when creating a zvol.
These commits do not fix the volmode issue, but a separate FreeBSD issue is fixed where changing volmode after creation would not take effect until the next time the pool is imported or the zvol is renamed.

Description

• Tidy up asserts (FreeBSD & Linux):

  • Using more specific assert variants gives better messages on failure.

• Keep better track of open count in close (FreeBSD):

  • zvol_geom_close gets a count of the number of close operations to do. Make sure we're always using this count to check if this will be the last close operation performed on the zvol.

• Code cleanup in zvol_create_minor_impl (FreeBSD):

  • Nonfunctional changes for readability and consistency.

• Properly ignore error in volmode lookup (FreeBSD):

  • We fall back to a default volmode and continue when looking up a zvol's volmode property fails. After this we should set the error to 0 to ensure we take the success paths in the out section.
  • While here, make sure we only log that the zvol was created on success.

• Don't leak doi in cdev error path (FreeBSD):

  • Make sure to free doi in zvol_create_minor impl when make_dev_s fails.

• Fix handling of zvol private data (FreeBSD):

  • zvol private data is supposed to be nulled by zvol_clear_private before zvol_free is called as an indicator that the zvol is going away.
  • Implement zvol_clear_private for volmode=dev.
  • Assert that zvol_clear_private has been called before zvol_free.
  • Check that zvol_clear_private has not been called when updating volsize. If it has, fail with ENXIO.

How Has This Been Tested?

ZTS on FreeBSD; manual messing with the volmode property

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 OpenZFS 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 #11117 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #11117   +/-   ##
=======================================
  Coverage   79.81%   79.82%           
=======================================
  Files         398      398           
  Lines      125754   125757    +3     
=======================================
+ Hits       100367   100381   +14     
+ Misses      25387    25376   -11     

Flag	Coverage Δ
#kernel	80.44% <100.00%> (-0.04%)	⬇️
#user	66.30% <ø> (+0.11%)	⬆️

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

Impacted Files	Coverage Δ
module/os/linux/zfs/zvol_os.c	87.44% <100.00%> (-0.81%)	⬇️
cmd/zdb/zdb_il.c	30.86% <0.00%> (-24.08%)	⬇️
module/zfs/vdev_indirect.c	73.92% <0.00%> (-10.97%)	⬇️
module/zfs/spa_errlog.c	90.83% <0.00%> (-3.06%)	⬇️
lib/libzfs/libzfs_changelist.c	84.44% <0.00%> (-1.86%)	⬇️
module/zfs/zfs_fm.c	85.80% <0.00%> (-1.75%)	⬇️
module/zfs/vdev_raidz.c	93.15% <0.00%> (-0.77%)	⬇️
module/zfs/vdev_mirror.c	94.44% <0.00%> (-0.75%)	⬇️
module/zfs/btree.c	83.02% <0.00%> (-0.71%)	⬇️
... and 42 more

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