Illumos #755: dmu_recv_stream builds incomplete guid_to_ds_map

Illumos #755: dmu_recv_stream builds incomplete guid_to_ds_map

An incomplete guid_to_ds_map would cause restore_write_byref() to fail
while receiving a de-duplicated backup stream.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Garrett D`Amore <garrett@nexenta.com>
Reviewed by: Gordon Ross <gwr@nexenta.com>
Approved by: Gordon Ross <gwr@nexenta.com>

References to Illumos issue and patch:
- https://www.illumos.org/issues/755
- https://github.com/illumos/illumos-gate/commit/ec5cf9d53a

Signed-off-by: Gunnar Beutner <gunnar@beutner.name>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #372