Prevent checkpointed DTLs from restarting resilvers

[favilances]

Motivation and Context

Pool checkpoints intentionally preserve DTL_MISSING entries after a scan because checkpoint-only blocks are not traversed. That retained DTL is needed if the pool is rewound, but it does not mean a new resilver was deferred.

Deferred resilvers rely on vdev_resilver_deferred to mark vdevs that missed txgs outside the active scan range. Treating any remaining DTL as deferred work breaks that distinction, so a completed resilver with a checkpoint can immediately queue another resilver and repeat indefinitely.

This fixes #11434 and fixes #17109. It follows the deferred-resilver intent from #7732 and keeps the restart avoidance added by #9588: a follow-up scan is only needed when a vdev was actually deferred.

Description

The follow-up resilver request is now gated on whether a leaf vdev had vdev_resilver_deferred set before it was cleared. Checkpoint-retained DTLs remain intact, while real deferred resilvers still run after the current scan completes.

A regression test creates a checkpoint during an attach resilver, lets the resilver finish, and verifies that no second resilver_start event is generated.

How Has This Been Tested?

• scripts/cstyle.pl module/zfs/vdev.c
• git diff --check
• bash -n tests/zfs-tests/tests/functional/replacement/resilver_restart_003.ksh
• scripts/commitcheck.sh HEAD

I did not run the ZFS Test Suite in this environment because this checkout is not configured (Makefile and config.status are absent) and ksh is not installed.

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.