ZTS: Fix zfs_mount.kshlib cleanup

[behlendorf]

Motivation and Context

Resolve occasional failures observed when running the ZTS zfs_mount group.

Description

Update cleanup_filesystem to use destroy_dataset when performing cleanup. This ensures the destroy is retried if the pool is busy preventing occasional failures.

How Has This Been Tested?

Locally running 20 iterations of the zfs_mount group resulted in a handful of failures caused by a zvol being busy at cleanup time. After applying this patch 100 iterations were run and no failures were observed. The logs showed that the retry code was invoked on several occasions preventing what otherwise would have been a test failure.

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.

[gmelikov]

LGTM, looks like we need to restart tests when github stabilizes.

[codecov-commenter]

Codecov Report

Merging #10358 into master will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10358      +/-   ##
==========================================
+ Coverage   79.38%   79.53%   +0.14%     
==========================================
  Files         391      391              
  Lines      123590   123590              
==========================================
+ Hits        98114    98296     +182     
+ Misses      25476    25294     -182     

Flag	Coverage Δ
#kernel	79.96% <ø> (+0.04%)	⬆️
#user	66.09% <ø> (+1.30%)	⬆️

Impacted Files	Coverage Δ
cmd/zvol_id/zvol_id_main.c	76.31% <0.00%> (-5.27%)	⬇️
module/zfs/bpobj.c	86.86% <0.00%> (-4.29%)	⬇️
module/lua/lmem.c	83.33% <0.00%> (-4.17%)	⬇️
module/zcommon/zfs_fletcher.c	75.65% <0.00%> (-2.64%)	⬇️
module/os/linux/zfs/vdev_file.c	80.37% <0.00%> (-1.87%)	⬇️
module/zfs/zio.c	87.18% <0.00%> (-1.79%)	⬇️
cmd/zed/agents/fmd_api.c	88.61% <0.00%> (-1.78%)	⬇️
module/zfs/vdev_raidz.c	91.39% <0.00%> (-1.75%)	⬇️
module/zfs/btree.c	81.85% <0.00%> (-1.64%)	⬇️
lib/libzfs/libzfs_changelist.c	85.15% <0.00%> (-1.18%)	⬇️
... and 49 more

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