net: generalize skb_segment()

While implementing GSO/TSO support for IPIP, I found skb_segment()
was assuming network header was immediately following mac header.

Its not really true in the case inet_gso_segment() is stacked :
By the time tcp_gso_segment() is called, network header points
to the inner IP header.

Let's instead assume nothing and pick the current offsets found in
original skb, we have skb_headers_offset_update() helper for that.

Also move the csum_start update inside skb_headers_offset_update()

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>