ip-header id needs to be incremented even if IP_DF flag is set.
This behaviour was changed in commit 490ab081
(IP_GRE: Fix IP-Identification).

Following patch fixes it so that identification is always
incremented.
Reported-by: NCong Wang <amwang@redhat.com>
Acked-by: NCong Wang <amwang@redhat.com>
Signed-off-by: NPravin B Shelar <pshelar@nicira.com>