udp: increase inner ip header ID during segmentation

Similar to GRE tunnel, UDP tunnel should take care of IP header ID
too.

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>
Signed-off-by: David S. Miller <davem@davemloft.net>