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/net/ipv6/addrconf_core.c b/net/ipv6/addrconf_core.c
index 2fe754f..5cd0029 100644
--- a/net/ipv6/addrconf_core.c
+++ b/net/ipv6/addrconf_core.c
@@ -161,12 +161,20 @@ eafnosupport_fib6_multipath_select(const struct net *net, struct fib6_info *f6i,
 	return f6i;
 }
 
+static u32
+eafnosupport_ip6_mtu_from_fib6(struct fib6_info *f6i, struct in6_addr *daddr,
+			       struct in6_addr *saddr)
+{
+	return 0;
+}
+
 const struct ipv6_stub *ipv6_stub __read_mostly = &(struct ipv6_stub) {
 	.ipv6_dst_lookup   = eafnosupport_ipv6_dst_lookup,
 	.fib6_get_table    = eafnosupport_fib6_get_table,
 	.fib6_table_lookup = eafnosupport_fib6_table_lookup,
 	.fib6_lookup       = eafnosupport_fib6_lookup,
 	.fib6_multipath_select = eafnosupport_fib6_multipath_select,
+	.ip6_mtu_from_fib6 = eafnosupport_ip6_mtu_from_fib6,
 };
 EXPORT_SYMBOL_GPL(ipv6_stub);