7247 zfs receive of deduplicated stream fails

This resolves two 'zfs recv' issues. First, when receiving into an
existing filesystem, a snapshot created during the receive process is
not added to the guid->dataset map for the stream, resulting in failed
lookups for deduped streams when a WRITE_BYREF record refers to a
snapshot received earlier in the stream. Second, the newly created
snapshot was also not set properly, referencing the snapshot before the
new receiving dataset rather than the existing filesystem.

Closes #159

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Dan Kimmel <dan.kimmel@delphix.com>
Author: Chris Williamson <chris.williamson@delphix.com>

openzfs/openzfs@b09697c8c18be68abfe538de9809938239402ae8