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);

include/net/xfrm.h

@@ -1608,6 +1608,7 @@ int xfrm6_find_1stfragopt(struct xfrm_state *x, struct sk_buff *skb,
 			  u8 **prevhdr);
 
 #ifdef CONFIG_XFRM
+void xfrm6_local_rxpmtu(struct sk_buff *skb, u32 mtu);
 int xfrm4_udp_encap_rcv(struct sock *sk, struct sk_buff *skb);
 int xfrm6_udp_encap_rcv(struct sock *sk, struct sk_buff *skb);
 int xfrm_user_policy(struct sock *sk, int optname,

net/ipv6/af_inet6.c

@@ -963,6 +963,7 @@ static const struct ipv6_stub ipv6_stub_impl = {
 	.udpv6_encap_enable = udpv6_encap_enable,
 	.ndisc_send_na = ndisc_send_na,
 #if IS_ENABLED(CONFIG_XFRM)
+	.xfrm6_local_rxpmtu = xfrm6_local_rxpmtu,
 	.xfrm6_udp_encap_rcv = xfrm6_udp_encap_rcv,
 	.xfrm6_rcv_encap = xfrm6_rcv_encap,
 #endif

net/ipv6/xfrm6_output.c

@@ -40,7 +40,7 @@ static int xfrm6_local_dontfrag(struct sk_buff *skb)
 	return 0;
 }
 
-static void xfrm6_local_rxpmtu(struct sk_buff *skb, u32 mtu)
+void xfrm6_local_rxpmtu(struct sk_buff *skb, u32 mtu)
 {
 	struct flowi6 fl6;
 	struct sock *sk = skb->sk;