network: set default priority for IPv6 routes

See inet6_rtm_newroute() in kernel's net/ipv6/route.c.

src/network/networkd-route.c

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
 #include <linux/icmpv6.h>
+#include <linux/ipv6_route.h>
 
 #include "alloc-util.h"
 #include "netlink-util.h"
@@ -2200,6 +2201,9 @@ static int route_section_verify(Route *route, Network *network) {
                         route->scope = RT_SCOPE_LINK;
         }
 
+        if (route->family == AF_INET6 && route->priority == 0)
+                route->priority = IP6_RT_PRIO_USER;
+
         if (ordered_hashmap_isempty(network->addresses_by_section) &&
             in_addr_is_null(route->family, &route->gw) == 0 &&
             route->gateway_onlink < 0) {