dpif-netdev: Optimize rwlock in port query API.

The query port only needs to use a rdlock, not a rwlock.
This patch can optimize it.

Fixes: 7daa503 ("dpif-netdev: Replace port mutex by rwlock.")
Acked-by: Mike Pattrick <mkp@redhat.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

Author: wyjwang

lib/dpif-netdev.c

@@ -2415,7 +2415,7 @@ dpif_netdev_port_query_by_number(const struct dpif *dpif, odp_port_t port_no,
     struct dp_netdev_port *port;
     int error;
 
-    ovs_rwlock_wrlock(&dp->port_rwlock);
+    ovs_rwlock_rdlock(&dp->port_rwlock);
     error = get_port_by_number(dp, port_no, &port);
     if (!error && dpif_port) {
         answer_port_query(port, dpif_port);