arp: Convert SIOCDARP and SIOCSARP to per-netns RTNL.

ioctl(SIOCDARP/SIOCSARP) operates on a single netns fetched from
an AF_INET socket in inet_ioctl().

Let's hold rtnl_net_lock() for SIOCDARP and SIOCSARP.

Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Reviewed-by: Eric Dumazet <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

net/ipv4/arp.c

···
 		IPV4_DEVCONF_ALL(net, PROXY_ARP) = on;
 		return 0;
 	}
-	if (__in_dev_get_rtnl(dev)) {
-		IN_DEV_CONF_SET(__in_dev_get_rtnl(dev), PROXY_ARP, on);
+	if (__in_dev_get_rtnl_net(dev)) {
+		IN_DEV_CONF_SET(__in_dev_get_rtnl_net(dev), PROXY_ARP, on);
 		return 0;
 	}
 	return -ENXIO;
···
 
 	switch (cmd) {
 	case SIOCDARP:
-		rtnl_lock();
+		rtnl_net_lock(net);
 		err = arp_req_delete(net, &r);
-		rtnl_unlock();
+		rtnl_net_unlock(net);
 		break;
 	case SIOCSARP:
-		rtnl_lock();
+		rtnl_net_lock(net);
 		err = arp_req_set(net, &r);
-		rtnl_unlock();
+		rtnl_net_unlock(net);
 		break;
 	case SIOCGARP:
 		rcu_read_lock();