openvswitch: Remove redundant key ref from upcall_info.

struct dp_upcall_info has pointer to pkt_key which is already
available in OVS_CB.  This also simplifies upcall handling
for gso packet.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>