zfs: fixes for a full stream received into an existing dataset

• this should fail early unless the force flag is set
• if the force flag is set then any local modifications including snapshots should be undone

See: https://www.illumos.org/issues/5912
See: https://reviews.csiden.org/r/220/

Reviewed by: mahrens, Paul Dagnelie <pcd@delphix.com>
MFC after: 15 days
Sponsored by: ClusterHQ