DDT: Move logs searches out of the lock

[amotin]

Postponing entry removal from the DDT log in case of hit till later single-threaded sync stage allows to make ddl_tree stable during multi-threaded ZIO processing stage. It allows to drop the DDT lock before the search instead of after, reducing the contention a lot.

Actually ddt_log_update_entry() was already handling the case of entry present in the active log without removal, so we only need to remove it from flushing log, if the entry happen to be there.

How Has This Been Tested?

My tests with parallel 4KB block writes show throughput increase from 480MB/s (122K blocks/s) to 827MB/s (212K blocks/s), even though still limited by the global DDT lock contention.

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.