ipv4: Fix ip-header identification for gso packets.

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: Cong Wang <amwang@redhat.com>
Acked-by: Cong Wang <amwang@redhat.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>