ZTS: Skip udev actions in zvol_misc when not Linux

[ghost]

Motivation and Context

udev is only used on Linux.

Description

Skip udev_wait and udev_cleanup when not on Linux.

How Has This Been Tested?

ZTS on FreeBSD

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 #10165 into master will decrease coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10165      +/-   ##
==========================================
- Coverage   79.40%   79.28%   -0.13%     
==========================================
  Files         385      385              
  Lines      122427   122427              
==========================================
- Hits        97215    97068     -147     
- Misses      25212    25359     +147     

Flag	Coverage Δ
#kernel	79.62% <ø> (-0.10%)	⬇️
#user	66.04% <ø> (-0.13%)	⬇️

Impacted Files	Coverage Δ
module/os/linux/spl/spl-kmem-cache.c	75.22% <0.00%> (-9.77%)	⬇️
module/zfs/vdev_raidz.c	88.99% <0.00%> (-4.69%)	⬇️
module/zfs/vdev_raidz_math.c	76.57% <0.00%> (-2.26%)	⬇️
module/zcommon/zfs_uio.c	90.52% <0.00%> (-2.11%)	⬇️
module/zfs/arc.c	81.51% <0.00%> (-1.78%)	⬇️
module/zfs/zrlock.c	89.23% <0.00%> (-1.54%)	⬇️
module/zfs/dmu_tx.c	82.84% <0.00%> (-1.19%)	⬇️
module/os/linux/zfs/zio_crypt.c	79.97% <0.00%> (-1.11%)	⬇️
module/os/linux/zfs/vdev_disk.c	84.00% <0.00%> (-1.10%)	⬇️
module/zfs/spa_misc.c	89.36% <0.00%> (-1.01%)	⬇️
... and 45 more

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