openvswitch: Optimize updating for OvS flow_stats.

In the ovs_flow_stats_update(), we only use the node
var to alloc flow_stats struct. But this is not a
common case, it is unnecessary to call the numa_node_id()
everytime. This patch is not a bugfix, but there maybe
a small increase.

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>