openvswitch: Fix GSO with multiple MPLS label.

MPLS GSO needs to know inner most protocol to process GSO packets.

Fixes: 25cd9ba0 ("openvswitch: Add basic MPLS support to
kernel").

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