ip_gre: increase inner ip header ID during segmentation

According to the previous discussion [1] on netdev list, DaveM insists
we should increase the IP header ID for each segmented packets.
This patch fixes it.

Cc: Pravin B Shelar <pshelar@nicira.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
1. http://marc.info/?t=136384172700001&r=1&w=2
Signed-off-by: David S. Miller <davem@davemloft.net>