net: add and use skb_gso_transport_seglen()

This moves part of Eric Dumazets skb_gso_seglen helper from tbf sched to
skbuff core so it may be reused by upcoming ip forwarding path patch.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/linux/skbuff.h
Simple merge

diff --cc net/core/skbuff.c
Simple merge

diff --cc net/sched/sch_tbf.c
Simple merge