fix: zfs-initramfs breakages after recent refactor

[lowjoel]

Motivation and Context

I was porting my zsys patches on my PPA and was running the resulting code changes by Claude for a review. Some changes were only in my patch, others were found to be in the upstream repository. Pushing those fixes here. I've also added the Co-authored-by tag (hopefully in compliance with policy). The problems were identified by Claude but I did the fixes by hand.

Description

See individual commits. They fix some identifier mix-ups in 61ab032 and 33dd57e.

This should also be a candidate for backporting to 2.4.x

How Has This Been Tested?

Mostly by static analysis of the comments and context. I've rebooted my machine with the new initramfs and cursorily it seems to work. Arguably, that won't tell us what we already know -- that this only affects the rollback path.

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS 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.

[Copilot]

Pull request overview

Fixes initramfs ZFS boot script regressions introduced by recent refactors, primarily around using the correct identifiers when destroying datasets and when prompting for a snapshot during snapshot-boot flows.

Changes:

• Fix destroy_fs() to consistently use its local _destroy_fs argument when logging and constructing the destroy command.
• Fix snapshot selection to update _boot_snap (the variable actually used later) when prompting the user.
• Fix dataset destruction loop to destroy each listed dataset (fs) rather than repeatedly destroying _boot_snap.

Comments suppressed due to low confidence (1)

contrib/initramfs/scripts/zfs:681

• The comment about splitting the snapshot still refers to ${snap}, but the actual variable used here is _boot_snap. Updating the comment (and fixing “it's”→“its”) would avoid confusion when maintaining this function.

		_boot_snap="$(ask_user_snap "${_boot_snap%%@*}")"
	fi

	# Separate the full snapshot ('${snap}') into it's filesystem and
	# snapshot names. Would have been nice with a split() function..

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[lowjoel]

zloop failures hopefully are unrelated: don't believe an initramfs change can change zloop results

[behlendorf]

Thanks for fixing this up. Yeah, the zloop failure is unrelated.