xfrm: move xfrm4_extract_header to common helper

The function only initializes the XFRM CB in the skb.

After previous patch xfrm4_extract_header is only called from
net/xfrm/xfrm_{input,output}.c.

Because of IPV6=m linker errors the ipv6 equivalent
(xfrm6_extract_header) was already placed in xfrm_inout.h because
we can't call functions residing in a module from the core.

So do the same for the ipv4 helper and place it next to the ipv6 one.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

include/net/xfrm.h

@@ -1562,7 +1562,6 @@ int pktgen_xfrm_outer_mode_output(struct xfrm_state *x, struct sk_buff *skb);
 #endif
 
 void xfrm_local_error(struct sk_buff *skb, int mtu);
-int xfrm4_extract_header(struct sk_buff *skb);
 int xfrm4_extract_input(struct xfrm_state *x, struct sk_buff *skb);
 int xfrm4_rcv_encap(struct sk_buff *skb, int nexthdr, __be32 spi,
 		    int encap_type);

net/ipv4/xfrm4_state.c

@@ -8,28 +8,7 @@
  *
  */
 
-#include <net/ip.h>
 #include <net/xfrm.h>
-#include <linux/pfkeyv2.h>
-#include <linux/ipsec.h>
-#include <linux/netfilter_ipv4.h>
-#include <linux/export.h>
-
-int xfrm4_extract_header(struct sk_buff *skb)
-{
-	const struct iphdr *iph = ip_hdr(skb);
-
-	XFRM_MODE_SKB_CB(skb)->ihl = sizeof(*iph);
-	XFRM_MODE_SKB_CB(skb)->id = iph->id;
-	XFRM_MODE_SKB_CB(skb)->frag_off = iph->frag_off;
-	XFRM_MODE_SKB_CB(skb)->tos = iph->tos;
-	XFRM_MODE_SKB_CB(skb)->ttl = iph->ttl;
-	XFRM_MODE_SKB_CB(skb)->optlen = iph->ihl * 4 - sizeof(*iph);
-	memset(XFRM_MODE_SKB_CB(skb)->flow_lbl, 0,
-	       sizeof(XFRM_MODE_SKB_CB(skb)->flow_lbl));
-
-	return 0;
-}
 
 static struct xfrm_state_afinfo xfrm4_state_afinfo = {
 	.family			= AF_INET,

net/xfrm/xfrm_inout.h

@@ -6,6 +6,20 @@
 #ifndef XFRM_INOUT_H
 #define XFRM_INOUT_H 1
 
+static inline void xfrm4_extract_header(struct sk_buff *skb)
+{
+	const struct iphdr *iph = ip_hdr(skb);
+
+	XFRM_MODE_SKB_CB(skb)->ihl = sizeof(*iph);
+	XFRM_MODE_SKB_CB(skb)->id = iph->id;
+	XFRM_MODE_SKB_CB(skb)->frag_off = iph->frag_off;
+	XFRM_MODE_SKB_CB(skb)->tos = iph->tos;
+	XFRM_MODE_SKB_CB(skb)->ttl = iph->ttl;
+	XFRM_MODE_SKB_CB(skb)->optlen = iph->ihl * 4 - sizeof(*iph);
+	memset(XFRM_MODE_SKB_CB(skb)->flow_lbl, 0,
+	       sizeof(XFRM_MODE_SKB_CB(skb)->flow_lbl));
+}
+
 static inline void xfrm6_extract_header(struct sk_buff *skb)
 {
 #if IS_ENABLED(CONFIG_IPV6)