5809 Blowaway full receive in v1 pool causes kernel panic

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Alex Reece <alex@delphix.com>
Reviewed by: Will Andrews <will@freebsd.org>
Approved by: Gordon Ross <gwr@nexenta.com>

usr/src/uts/common/fs/zfs/dmu_send.c

@@ -1184,7 +1184,8 @@ dmu_recv_begin_sync(void *arg, dmu_tx_t *tx)
 		}
 		dsobj = dsl_dataset_create_sync(ds->ds_dir, recv_clone_name,
 		    snap, crflags, drba->drba_cred, tx);
-		dsl_dataset_rele(snap, FTAG);
+		if (drba->drba_snapobj != 0)
+			dsl_dataset_rele(snap, FTAG);
 		dsl_dataset_rele(ds, FTAG);
 	} else {
 		dsl_dir_t *dd;