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
Cc: Steffen Klassert
Cc: "David S. Miller"
Cc: Herbert Xu
Signed-off-by: David S. Miller