openvswitch: Fix Frame-size larger than 1024 bytes warning.

There is no need to declare separate key on stack,
we can just use sw_flow->key to store the key directly.

This commit fixes following warning:

net/openvswitch/datapath.c: In function ‘ovs_flow_cmd_new’:
net/openvswitch/datapath.c:1080:1: warning: the frame size of 1040 bytes
is larger than 1024 bytes [-Wframe-larger-than=]

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