net: tso: cache transport header length

Add tlen field into struct tso_t, and change tso_start()
to return skb_transport_offset(skb) + tso->tlen

This removes from callers the need to use tcp_hdrlen(skb) and
will ease UDP segmentation offload addition.

v2: calls tso_start() earlier in otx2_sq_append_tso() [Jakub]

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