openvswitch: Set flow-key members.

This patch adds missing memset which are required to initialize
flow key member. For example for IP flow we need to initialize
ip.frag for all cases.

Found by inspection.

This bug is introduced by commit 07148121
("openvswitch: Eliminate memset() from flow_extract").

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