systemd: serialize generated initrd fsck device probes

[Pawel Winogrodzki (PawelWMS)]

What

• Patch systemd-fstab-generator so generated initrd root and /usr filesystem checks run through udevadm lock.
• Use the exact generated device path for lock acquisition and systemd-fsck.
• Update existing generator fixtures and add coverage for generated /usr filesystem checks.

Why

systemd-udevd can probe filesystem metadata while fsck is still updating it. Serializing the writer against udev's shared whole-disk lock prevents udev from observing transient metadata and removing filesystem identity links before the mount begins.

Scope

This option targets only generator-synthesized initrd systemd-fsck-root.service and systemd-fsck-usr.service units. Static root and instantiated post-switch-root checks remain unchanged. Draft #18639 explores the complementary static drop-in option.

Risk

• The behavior is unconditional after the package upgrade and initramfs regeneration; rollback requires removing the patch and rebuilding the initramfs.
• Lock acquisition has a hard 180-second ceiling; timeout prevents fsck from starting and fails the generated unit.
• Root and /usr partitions on one physical disk serialize instead of checking concurrently.
• This downstream source patch must be rebased when systemd changes the generator or fixtures.

Verification

• Verified the patch applies to the pristine systemd v255 source used by this package.
• Updated all affected test-fstab-generator fixtures and added generated /usr coverage.
• Ran git diff --check.
• Passed repository commit and push hooks.
• Completed adversarial Advocate, Skeptic, and Architect review.
• Full package build and generator test execution were not available locally; draft CI and review are required.