ipip: Use tunnel_ip_select_ident() for tunnel IP-Identification.

tunnel_ip_select_ident() is more efficient when generating ip-header
id given inner packet is of ipv4 type.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>