Avoid waiting in dmu_sync_late_arrival().

[amotin]

The transaction there does not produce any dirty data or log blocks, so it should not be throttled. All other cases wait for TXG sync, by which time the log block we are writing will be obsolete, so we can skip waiting and just return error here instead.

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.

[behlendorf]

The explanation here makes sense, however the test results tell a different story. Multiple builders hung after the slog_004_pos test case which is highly suspicious.

[amotin]

The explanation here makes sense, however the test results tell a different story. Multiple builders hung after the slog_004_pos test case which is highly suspicious.

The only backtraces we found look identical to ones fixed by #15103 .

[behlendorf]

Things appear to be in much better shape after rebasing on the #15103 fix.