DDT: Reduce global DDT lock scope during writes

[amotin]

Before this change DDT lock was taken 4 times from two different threads per written block, and as effectively a pool-wide lock it can be highly congested. This change introduces a new per-entry dde_io_lock, protecting some fields during I/O ready and done stages, so that we don't need the global lock there, leaving 2 acquisitions from one thread per written block.

According to my write tests on 64-thread system with 4KB blocks this significantly reduce the global lock contention, reducing CPU usage from 100% to expected ~80%, and increasing write throughput by 10%.

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.