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/af_inet6.c b/net/ipv6/af_inet6.c
index 50de8b0..9ed0eae 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -894,6 +894,7 @@ static const struct ipv6_stub ipv6_stub_impl = {
 	.fib6_table_lookup = fib6_table_lookup,
 	.fib6_lookup       = fib6_lookup,
 	.fib6_multipath_select = fib6_multipath_select,
+	.ip6_mtu_from_fib6 = ip6_mtu_from_fib6,
 	.udpv6_encap_enable = udpv6_encap_enable,
 	.ndisc_send_na = ndisc_send_na,
 	.nd_tbl	= &nd_tbl,