ZIL: Improve write log size accounting

[amotin]

Before this change write log size TXG throttling mechanism was accounting only user payload bytes. But the actual ZIL both on disk and especially in memory include headers of hundred(s) of bytes. Not accouting those may allow applications like bonnie++, in their wisdom writing one byte at a time, to consume excessive amount of memory and ZIL/SLOG in one TXG.

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.

[robn]

Very good.