MFC r336961:
MFV r336960: 9256 zfs send space estimation off by > 10% on some datasets

illumos/illummos-gate@df477c0afa111b5205c872dab36dbfde391656de

Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: John Kennedy <john.kennedy@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Paul Dagnelie <pcd@delphix.com>