The send size estimate for a zvol can be too low, if the size of the record
headers (dmu_replay_record_t's) is a significant portion of the size.
This is typically the case when the data is highly compressible, especially
with embedded blocks.
The problem is that dmu_adjust_send_estimate_for_indirects() assumes that
blocks are the size of the "recordsize" property (128KB).
However, for zvols, the blocks are the size of the "volblocksize" property
(8KB). Therefore, we estimate that there will be 16x less record headers than
there really will be.