net: openvswitch: use hlist_for_each_entry_rcu instead of hlist_for_each_entry

The struct sw_flow is protected by RCU, when traversing them,
use hlist_for_each_entry_rcu.

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Tested-by: Greg Rose <gvrose8192@gmail.com>
Reviewed-by: Greg Rose <gvrose8192@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>