MFV r294796: 6319 assertion failed in zio_ddt_write: bp->blk_birth == txg

Reviewed by: George Wilson <george.wilson@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

illumos/illumos-gate@b39b744be78c6327db43c1f69d11c2f5909f73cb

This is revert of 5693.