net: warn if transport header was not set

Make sure skb_transport_header() and skb_transport_offset() uses
are not fooled if the transport header has not been set.

This change will likely expose existing bugs in linux networking stacks.

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