xfrm: place xfrm6_local_dontfrag in xfrm.h

so next patch can re-use it from net/xfrm/xfrm_output.c without
causing a linker error when IPV6 is a module.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/net/xfrm.h

@@ -1993,4 +1993,20 @@ static inline int xfrm_tunnel_check(struct sk_buff *skb, struct xfrm_state *x,
 
 	return 0;
 }
+
+#if IS_ENABLED(CONFIG_IPV6)
+static inline bool xfrm6_local_dontfrag(const struct sock *sk)
+{
+	int proto;
+
+	if (!sk || sk->sk_family != AF_INET6)
+		return false;
+
+	proto = sk->sk_protocol;
+	if (proto == IPPROTO_UDP || proto == IPPROTO_RAW)
+		return inet6_sk(sk)->dontfrag;
+
+	return false;
+}
+#endif
 #endif	/* _NET_XFRM_H */

net/ipv6/xfrm6_output.c

@@ -23,23 +23,6 @@ int xfrm6_find_1stfragopt(struct xfrm_state *x, struct sk_buff *skb,
 }
 EXPORT_SYMBOL(xfrm6_find_1stfragopt);
 
-static int xfrm6_local_dontfrag(struct sk_buff *skb)
-{
-	int proto;
-	struct sock *sk = skb->sk;
-
-	if (sk) {
-		if (sk->sk_family != AF_INET6)
-			return 0;
-
-		proto = sk->sk_protocol;
-		if (proto == IPPROTO_UDP || proto == IPPROTO_RAW)
-			return inet6_sk(sk)->dontfrag;
-	}
-
-	return 0;
-}
-
 void xfrm6_local_rxpmtu(struct sk_buff *skb, u32 mtu)
 {
 	struct flowi6 fl6;
@@ -82,7 +65,7 @@ static int xfrm6_tunnel_check_size(struct sk_buff *skb)
 		skb->dev = dst->dev;
 		skb->protocol = htons(ETH_P_IPV6);
 
-		if (xfrm6_local_dontfrag(skb))
+		if (xfrm6_local_dontfrag(skb->sk))
 			xfrm6_local_rxpmtu(skb, mtu);
 		else if (skb->sk)
 			xfrm_local_error(skb, mtu);
@@ -181,7 +164,7 @@ static int __xfrm6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 
 	toobig = skb->len > mtu && !skb_is_gso(skb);
 
-	if (toobig && xfrm6_local_dontfrag(skb)) {
+	if (toobig && xfrm6_local_dontfrag(skb->sk)) {
 		xfrm6_local_rxpmtu(skb, mtu);
 		kfree_skb(skb);
 		return -EMSGSIZE;