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>