Export symbol dmu_tx_mark_netfree()

[behlendorf]

Description

This symbol is needed by Lustre and should have been exported
when the original patch was merged.

Motivation and Context

Lustre needs to be able to mark TXs as net free the same way the ZPL does.
https://review.whamcloud.com/#/c/26930/

How Has This Been Tested?

Buildbot.

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.
• All new and existing tests passed.
• All commit messages are properly formatted and contain Signed-off-by.
• Change has been approved by a ZFS on Linux member.

[behlendorf]

@bzzz77 can you please review this, you'll need it for https://review.whamcloud.com/#/c/26930/.

[codecov]

Codecov Report

Merging #6660 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #6660     +/-   ##
========================================
+ Coverage   67.39%   67.5%   +0.1%     
========================================
  Files         196     196             
  Lines       70356   70348      -8     
  Branches    13921   13921             
========================================
+ Hits        47419   47486     +67     
+ Misses      17517   17511      -6     
+ Partials     5420    5351     -69

Impacted Files	Coverage Δ
module/zfs/dmu_tx.c	70.2% <ø> (ø)	⬆️
module/zfs/sha256.c	52.38% <0%> (-19.05%)	⬇️
cmd/ztest/ztest.c	75.95% <0%> (-1.35%)	⬇️
module/zfs/multilist.c	96.42% <0%> (-0.9%)	⬇️
module/zfs/zio.c	60.01% <0%> (-0.81%)	⬇️
module/zfs/zfs_rlock.c	90.79% <0%> (-0.42%)	⬇️
module/zcommon/zfs_fletcher.c	66.54% <0%> (-0.36%)	⬇️
module/zfs/zpl_xattr.c	65.87% <0%> (-0.22%)	⬇️
module/zfs/dsl_dir.c	73.28% <0%> (-0.14%)	⬇️
module/zcommon/zfs_namecheck.c	29.16% <0%> (ø)	⬆️
... and 43 more

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