ipv6: check mc_forwarding before calling ip6mr_get_route()

When CONFIG_IPV6_MROUTE is enabled, rt6_fill_node() unconditionally calls ip6mr_get_route() whenever an input interface is specified and the destination address is multicast.

However, if multicast routing is not active (i.e. no multicast routing daemon is running and mc_forwarding is 0), there is no matching (S,G) multicast routing cache entry, and skb->dev is NULL in the synthetic route query skb allocated by inet6_rtm_getroute(). As a result, ip6mr_get_route() returns -ENODEV, which causes rt6_fill_node() to jump to nla_put_failure and return -EMSGSIZE to userspace ("RTNETLINK answers: Message too long").

In contrast, IPv4 counterpart rt_fill_info() checks IPV4_DEVCONF_ALL_RO(net, MC_FORWARDING) before attempting to query ipmr_get_route(), and falls back to emitting RTA_IIF and normal local delivery route attributes when multicast routing is not enabled.

This discrepancy was identified during code audit of multicast route get handling. The failure was confirmed on a UML kernel: querying an IPv6 multicast route with an input interface ("ip -6 route get <mcast_addr> from <src_addr> ...") failed with -EMSGSIZE on an unpatched kernel.

Align IPv6 behavior with IPv4 by checking whether mc_forwarding is active (atomic_read(&net->ipv6.devconf_all->mc_forwarding)) before calling ip6mr_get_route(). When mc_forwarding is 0, rt6_fill_node() proceeds to output RTA_IIF and normal route attributes.

Tested:
- On a UML kernel with CONFIG_IPV6_MROUTE=y, verified that
"ip -6 route get ff0e::1 from 2001:db8::3 ..." with an input interface now succeeds and outputs the route instead of failing with -EMSGSIZE.
- Verified that IPv6 unicast route get and multicast route get without an input interface continue to work as expected.
- Built with W=1 on UML, x86_64 allmodconfig, and x86_64 allyesconfig with zero new warnings or errors.

---
 net/ipv6/route.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

index 2eb884558c24..6dd4a70f6490 100644
@@ -5862,7 +5862,8 @@ static int rt6_fill_node(struct net *net, struct sk_buff *skb,
 #endif
 	if (iif) {
 #ifdef CONFIG_IPV6_MROUTE
-		if (ipv6_addr_is_multicast(&rt6_dst->addr)) {
+		if (ipv6_addr_is_multicast(&rt6_dst->addr) &&
+		    atomic_read(&net->ipv6.devconf_all->mc_forwarding)) {
 			int err = ip6mr_get_route(net, skb, rtm, portid);
 
 			if (err == 0)

2.33.0