Ensure initramfs directory /usr/bin exists before attempting subsequent copy

[IslandLife]

Motivation and Context

Fix for #10434 initramfs hook zfsunlock creates initramfs /usr/bin as as file and not directory

Description

Modeled on the similar mkdir statement for inirtramfs /etc in the hook zfs

How Has This Been Tested?

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)

Invoke update-initramfs

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]

Codecov Report

Merging #10448 into master will decrease coverage by 0.27%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #10448      +/-   ##
==========================================
- Coverage   79.58%   79.31%   -0.28%     
==========================================
  Files         391      391              
  Lines      123872   123872              
==========================================
- Hits        98586    98247     -339     
- Misses      25286    25625     +339     

Flag	Coverage Δ
#kernel	80.01% <ø> (-0.02%)	⬇️
#user	64.67% <ø> (-1.45%)	⬇️

Impacted Files	Coverage Δ
cmd/ztest/ztest.c	73.28% <0.00%> (-6.85%)	⬇️
module/zfs/vdev_indirect.c	74.66% <0.00%> (-6.00%)	⬇️
module/zfs/bpobj.c	86.86% <0.00%> (-4.29%)	⬇️
module/zfs/dsl_scan.c	82.92% <0.00%> (-3.01%)	⬇️
module/zcommon/zfs_fletcher.c	75.65% <0.00%> (-2.64%)	⬇️
module/zfs/vdev_removal.c	94.85% <0.00%> (-2.63%)	⬇️
module/zfs/dsl_synctask.c	92.40% <0.00%> (-2.54%)	⬇️
module/zfs/vdev_indirect_mapping.c	96.61% <0.00%> (-1.94%)	⬇️
module/icp/api/kcf_mac.c	37.14% <0.00%> (-1.72%)	⬇️
module/zfs/ddt.c	87.47% <0.00%> (-0.78%)	⬇️
... and 47 more

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[rlaager]

copy_exec calls copy_file, which has a mkdir in it. Instead of adding a mkdir, does this work:
copy_exec /usr/share/initramfs-tools/zfsunlock /usr/bin/zfsunlock

[behlendorf]

I agree, this would be better.

[behlendorf]

@IslandLife can you make suggested change to this PR so we can get it merged.

[behlendorf]

Closing. Resolved by #11162.