net/ipv6: Add helper to return path MTU based on fib result

Determine path MTU from a FIB lookup result. Logic is based on
ip6_dst_mtu_forward plus lookup of nexthop exception.

Add ip6_dst_mtu_forward to ipv6_stubs to handle access by core
bpf code.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
diff --git a/include/net/addrconf.h b/include/net/addrconf.h
index ff766ab..c07d4dd 100644
--- a/include/net/addrconf.h
+++ b/include/net/addrconf.h
@@ -236,6 +236,8 @@ struct ipv6_stub {
 						   struct flowi6 *fl6, int oif,
 						   const struct sk_buff *skb,
 						   int strict);
+	u32 (*ip6_mtu_from_fib6)(struct fib6_info *f6i, struct in6_addr *daddr,
+				 struct in6_addr *saddr);
 
 	void (*udpv6_encap_enable)(void);
 	void (*ndisc_send_na)(struct net_device *dev, const struct in6_addr *daddr,