xfrm: expose local_rxpmtu via ipv6_stubs

We cannot call this function from the core kernel unless we would force
CONFIG_IPV6=y.

Therefore expose this via ipv6_stubs so we can call it from net/xfrm
in the followup patch.

Since the call is expected to be unlikely, no extra code for the IPV6=y
case is added and we will always eat the indirection cost.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/net/ipv6_stubs.h

@@ -57,6 +57,7 @@ struct ipv6_stub {
 			      const struct in6_addr *solicited_addr,
 			      bool router, bool solicited, bool override, bool inc_opt);
 #if IS_ENABLED(CONFIG_XFRM)
+	void (*xfrm6_local_rxpmtu)(struct sk_buff *skb, u32 mtu);
 	int (*xfrm6_udp_encap_rcv)(struct sock *sk, struct sk_buff *skb);
 	int (*xfrm6_rcv_encap)(struct sk_buff *skb, int nexthdr, __be32 spi,
 			       int encap_type);