Fix zfs-mount-generator escaping

[rlaager]

Motivation and Context

This fixes the name of the mount unit for the root filesystem. I discovered these pre-existing issues in review/testing of #9649.

In practice, the root filesystem is mounted by the initrd anyway, so this is moot in practice (which is why this wasn't noticed), but we should be correct nonetheless.

Description

The correct name for the mount unit for / is "-.mount", not ".mount".

How Has This Been Tested?

I used some print statements and some diffing. This is a nice test case, if it works on your particular system:

rm -rf /tmp/before
mkdir /tmp/before
sudo /lib/systemd/system-generators/zfs-mount-generator /tmp/before /tmp/before /tmp/before

rm -rf /tmp/after
mkdir /tmp/after
sudo /lib/systemd/system-generators/zfs-mount-generator /tmp/after /tmp/after /tmp/after

diff -urN /tmp/before /tmp/after

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.

[behlendorf]

This looks correct, but I did not manually test it.

[behlendorf]

@aerusso would you mind taking a look at this change.

[rlaager]

I have changed to use the more proper form of escaping, as suggested by @aerusso. This also eliminates a lot of the need for the first commit. I'm happy with this simpler change.

[rlaager]

@aerusso Since you basically "rewrote" this one-liner, I gave you a Co-authored-by credit here. I can make you the sole author if you'd prefer.

[codecov]

Codecov Report

Merging #9970 into master will increase coverage by <1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master    #9970    +/-   ##
========================================
+ Coverage      79%      79%   +<1%     
========================================
  Files         385      385            
  Lines      122382   122382            
========================================
+ Hits        96922    97092   +170     
+ Misses      25460    25290   -170

Flag	Coverage Δ
#kernel	79% <ø> (ø)	⬆️
#user	67% <ø> (ø)	⬆️

---

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 12f7b90...ba453cd. Read the comment docs.

[aerusso]

This looks good! The co-authorship is fine, too.