Fix race between resilver wait and offline/detach

[amotin]

We should not clear scn_state and notify waiters until we call vdev_dtl_reassess(), otherwise following offline/detach request may fail with "no valid replicas" error.

How Has This Been Tested?

I was unable to reproduce it by hands, but #17259 seems uncovered it on some of later online_offline_001_pos test runs: https://github.com/openzfs/zfs/actions/runs/14625988592/job/41037499459 , resulting in this error:

20:05:36.83 SUCCESS: zpool online testpool md0
20:05:36.84 SUCCESS: check_state testpool md0 online
20:05:36.85 SUCCESS: zpool wait -t resilver testpool
20:05:36.86 cannot offline md1: no valid replicas
20:05:36.86 ERROR: zpool offline testpool md1 exited 1

I hope this to fix it.

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)
• 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.

[amotin]

While I still think the fix is valid, it apparently fixes a different problem. After more code reading I've found another race window, caused by the fact that zpool online starts resilver asynchronously and zpool wait can't wait for it. I don't have enough motivation to start refactoring there, so just restore some delays I removed before to cover it for now.

[tonyhutter]

Can we get one more reviewer on this one?

[tonyhutter]

Merged as:
f85c96e ZTS: Restore some delays in online_offline tests
f86d9af Fix race between resilver wait and offline/detach