ZIO: ZIO_STAGE_DDT_WRITE is a blocking stage

[amotin]

ddt_lookup() in zio_ddt_write() might require synchronous DDT ZAP read. Running it from interrupt taskq might lead to deadlock. Inclusion of ZIO_STAGE_DDT_WRITE into ZIO_BLOCKING_STAGES should hopefully fix that, even though I am not sure how I got there.

Types of changes

• [x] 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:

• [x] 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.
• [x] All commit messages are properly formatted and contain Signed-off-by.