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

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/dmu_send.c

@@ -983,10 +983,12 @@ recv_begin_check_existing_impl(dmu_recv_begin_arg_t *drba, dsl_dataset_t *ds,
 
 		dsl_dataset_rele(snap, FTAG);
 	} else {
-		/* if full, most recent snapshot must be $ORIGIN */
-		if (dsl_dataset_phys(ds)->ds_prev_snap_txg >= TXG_INITIAL)
-			return (SET_ERROR(ENODEV));
-		drba->drba_snapobj = dsl_dataset_phys(ds)->ds_prev_snap_obj;
+		/* if full, then must be forced */
+		if (!drba->drba_cookie->drc_force)
+			return (SET_ERROR(EEXIST));
+		/* start from $ORIGIN@$ORIGIN, if supported */
+		drba->drba_snapobj = dp->dp_origin_snap != NULL ?
+		    dp->dp_origin_snap->ds_object : 0;
 	}
 
 	return (0);