In dmu_sync_ready(), a hole block pointer will have it's logical size
explicitly set as it's necessary for replay purposes. To "undo" this,
dmu_sync_done() will zero out any hole that it finds. This becomes a
problem when using the "hole_birth" feature, as this will also wipe out
any birth time that might have happened to be set on the hole.
...
As a fix, the logic to zero out a hole is only applied to old style
holes with a birth time of zero. Holes created with the "hole_birth"
feature enabled will have a non-zero birth time, and will be skipped
(thus preserving the ltime, type, and level information as well).
In addition, zdb was updated to also print the ltime, type, and level
information for these new style holes. Previously, only the logical
birth time would be printed.