ip_tunnel: Do not use inner ip-header-id for tunnel ip-header-id.

[ Upstream commit 4221f40513233fa8edeef7fc82e44163fde03b9b ]

Using inner-id for tunnel id is not safe in some rare cases.
E.g. packets coming from multiple sources entering same tunnel
can have same id. Therefore on tunnel packet receive we
could have packets from two different stream but with same
source and dst IP with same ip-id which could confuse ip packet
reassembly.

Following patch reverts optimization from commit
490ab081 (IP_GRE: Fix IP-Identification.)

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
CC: Jarno Rajahalme <jrajahalme@nicira.com>
CC: Ansis Atteka <aatteka@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>