7495 loader should boot pre-feature flags pools.

Reviewed by: Andrew Stormont <astormont@racktopsystems.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Matthew Ahrens <mahrens@delphix.com>

usr/src/boot/sys/boot/zfs/zfsimpl.c

@@ -2079,8 +2079,16 @@ check_mos_features(const spa_t *spa)
 	if ((rc = objset_get_dnode(spa, &spa->spa_mos, DMU_OT_OBJECT_DIRECTORY,
 	    &dir)) != 0)
 		return (rc);
-	if ((rc = zap_lookup(spa, &dir, DMU_POOL_FEATURES_FOR_READ, &objnum)) != 0)
+	if ((rc = zap_lookup(spa, &dir, DMU_POOL_FEATURES_FOR_READ,
+	    &objnum)) != 0) {
+		/*
+		 * It is older pool without features. As we have already
+		 * tested the label, just return without raising the error.
+		 */
+		if (rc == ENOENT)
+			rc = 0;
 		return (rc);
+	}
 
 	if ((rc = objset_get_dnode(spa, &spa->spa_mos, objnum, &dir)) != 0)
 		return (rc);