Illumos 5912 - full stream can not be force-received into a dataset if it has a snapshot

5912 full stream can not be force-received into a dataset if it has a snapshot
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Dan McDonald <danmcd@omniti.com>

References:
  https://www.illumos.org/issues/5912
  illumos/illumos-gate@5bae108

Ported-by: Andriy Gapon <avg@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3549

Author: avg-I

module/zfs/dmu_send.c

@@ -1036,10 +1036,12 @@ 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);