net: move pskb_put() to core code

This function has usage beside IPsec so move it to the core skbuff code.
While doing so, give it some documentation and change its return type to
'unsigned char *' to be in line with skb_put().

Signed-off-by: Mathias Krause <mathias.krause@secunet.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 2e153b6..491dd6c 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1417,6 +1417,7 @@
 /*
  *	Add data to an sk_buff
  */
+unsigned char *pskb_put(struct sk_buff *skb, struct sk_buff *tail, int len);
 unsigned char *skb_put(struct sk_buff *skb, unsigned int len);
 static inline unsigned char *__skb_put(struct sk_buff *skb, unsigned int len)
 {