Fix SC2086 note in zpool.d/smart

[ghost]

Motivation and Context

./cmd/zpool/zpool.d/smart:78:32:
note: Double quote to prevent globbing and word splitting. [SC2086]

Reported by latest shellcheck on FreeBSD.

Description

Add missing quotes around a variable usage in zpool.d/smart.

How Has This Been Tested?

gmake shellcheck with hs-ShellCheck-0.7.1 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 #10194 into master will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10194      +/-   ##
==========================================
- Coverage   79.48%   79.23%   -0.26%     
==========================================
  Files         387      387              
  Lines      123388   123388              
==========================================
- Hits        98079    97762     -317     
- Misses      25309    25626     +317     

Flag	Coverage Δ
#kernel	79.86% <ø> (-0.11%)	⬇️
#user	65.41% <ø> (-0.70%)	⬇️

Impacted Files	Coverage Δ
module/os/linux/spl/spl-zlib.c	55.35% <0.00%> (-28.58%)	⬇️
module/os/linux/spl/spl-kmem-cache.c	75.22% <0.00%> (-9.59%)	⬇️
module/zfs/vdev_indirect.c	75.00% <0.00%> (-9.00%)	⬇️
module/zfs/space_map.c	93.06% <0.00%> (-5.20%)	⬇️
cmd/ztest/ztest.c	76.36% <0.00%> (-4.43%)	⬇️
module/zfs/spa_checkpoint.c	93.78% <0.00%> (-4.35%)	⬇️
module/lua/lmem.c	83.33% <0.00%> (-4.17%)	⬇️
module/zfs/zio_compress.c	89.74% <0.00%> (-2.57%)	⬇️
module/zfs/dsl_synctask.c	92.40% <0.00%> (-2.54%)	⬇️
module/os/linux/zfs/vdev_file.c	80.37% <0.00%> (-1.87%)	⬇️
... and 53 more

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