net: adopt skb_network_header_len() more broadly

(skb_transport_header(skb) - skb_network_header(skb))
can be replaced by skb_network_header_len(skb)

Add a DEBUG_NET_WARN_ON_ONCE() in skb_network_header_len()
to catch cases were the transport_header was not set.

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