BRT: Make BRT block sizes configurable

[amotin]

Similar to DDT make BRT data and indirect block sizes configurable via module parameters. I am not sure what would be the best yet, but similar to DDT 4KB blocks kill all chances of compression on vdev with ashift=12 or more, that on my tests reaches 3x.

While here, fix documentation for respective DDT parameters.

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]

Yup, it'd be nice to at least make these tunable to make it easier to experiment.

[pjd]

If you are there, I don't think we prefetch before the free, which would be really nice to do before we enter syncing context.

[amotin]

@pjd In my benchmarks I've found that ZAP prefetch can be pretty expensive, so we should use it carefully, though understanding that without it it may become even worse if ZAP is not in cache. Plus brt_entry_prefetch() is often getting blocked on BRT lock, which is held as writer for significant amount of time by sync thread. So that area does need more attention, far out this PR scope.