net: add skb_[inner_]tcp_all_headers helpers

Most drivers use "skb_transport_offset(skb) + tcp_hdrlen(skb)"
to compute headers length for a TCP packet, but others
use more convoluted (but equivalent) ways.

Add skb_tcp_all_headers() and skb_inner_tcp_all_headers()
helpers to harmonize this a bit.

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