Fix ZVOL_DIR

[ghost]

Motivation and Context

We only use ZVOL_DIR on FreeBSD, and on FreeBSD it isn't even correct.

Description

Move the definition to the file where it is needed, and define it as
/dev/zvol/.

How Has This Been Tested?

Ran tests for the zvol tag 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 #10337 into master will decrease coverage by 0.49%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10337      +/-   ##
==========================================
- Coverage   79.88%   79.38%   -0.50%     
==========================================
  Files         292      390      +98     
  Lines       83481   123350   +39869     
==========================================
+ Hits        66686    97924   +31238     
- Misses      16795    25426    +8631     

Flag	Coverage Δ
#kernel	79.85% <ø> (-0.04%)	⬇️
#user	65.62% <ø> (?)

Impacted Files	Coverage Δ
module/zfs/zcp_set.c	87.09% <0.00%> (-12.91%)	⬇️
module/zfs/zfs_ratelimit.c	90.47% <0.00%> (-9.53%)	⬇️
module/zfs/dmu_redact.c	82.15% <0.00%> (-6.24%)	⬇️
module/zfs/zfs_byteswap.c	79.45% <0.00%> (-5.85%)	⬇️
module/zcommon/zfs_deleg.c	92.45% <0.00%> (-5.55%)	⬇️
module/zcommon/zfeature_common.c	92.85% <0.00%> (-4.82%)	⬇️
module/zfs/dsl_deleg.c	90.07% <0.00%> (-4.48%)	⬇️
module/zfs/zio_inject.c	75.80% <0.00%> (-4.41%)	⬇️
module/zfs/spa_checkpoint.c	93.78% <0.00%> (-4.26%)	⬇️
module/zfs/fm.c	36.77% <0.00%> (-4.10%)	⬇️
... and 278 more

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4d6043f...1cdc940. Read the comment docs.

[codecov-io]

Codecov Report

Merging #10337 into master will decrease coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10337      +/-   ##
==========================================
- Coverage   79.88%   79.64%   -0.24%     
==========================================
  Files         292      390      +98     
  Lines       83481   123350   +39869     
==========================================
+ Hits        66686    98241   +31555     
- Misses      16795    25109    +8314     

Flag	Coverage Δ
#kernel	79.99% <ø> (+0.11%)	⬆️
#user	66.32% <ø> (?)

Impacted Files	Coverage Δ
module/zfs/zcp_set.c	87.09% <0.00%> (-12.91%)	⬇️
module/zfs/zfs_ratelimit.c	90.47% <0.00%> (-9.53%)	⬇️
module/zfs/dmu_redact.c	82.15% <0.00%> (-6.24%)	⬇️
module/zfs/zfs_byteswap.c	79.45% <0.00%> (-5.85%)	⬇️
module/zcommon/zfs_deleg.c	92.45% <0.00%> (-5.55%)	⬇️
module/zcommon/zfeature_common.c	92.85% <0.00%> (-4.82%)	⬇️
module/zfs/dsl_deleg.c	90.07% <0.00%> (-4.48%)	⬇️
module/zfs/zio_inject.c	75.80% <0.00%> (-4.41%)	⬇️
module/zfs/spa_checkpoint.c	93.78% <0.00%> (-4.26%)	⬇️
module/zfs/fm.c	36.77% <0.00%> (-4.10%)	⬇️
... and 279 more

---

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 4d6043f...1cdc940. Read the comment docs.