ovs: align nlattr properly when needed

I also fix commit 8b32ab9e6ef1: use nla_total_size_64bit() for
OVS_FLOW_ATTR_USED in ovs_flow_cmd_msg_size().

Fixes: 8b32ab9e6ef1 ("ovs: use nla_put_u64_64bit()")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>