6513 partially filled holes lose birth time

illumos/illumos-gate@8df0bcf0df7622a075cc6e52f659d2fcfdd08cdc
https://github.com/illumos/illumos-gate/commit/8df0bcf0df7622a075cc6e52f659d2fcfdd08cdc

https://www.illumos.org/issues/6513

If a ZFS object contains a hole at level one, and then a data block is created
at level 0 underneath that l1 block, l0 holes will be created. However, these
l0 holes do not have the birth time property set; as a result, incremental
sends will not send those holes.
Fix is to modify the dbuf_read code to fill in birth time data.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Boris Protopopov <bprotopopov@hotmail.com>
Approved by: Richard Lowe <richlowe@richlowe.net>
Author: Paul Dagnelie <pcd@delphix.com>