net: Fix device name resolving crash in default_device_exit()

The following script makes kernel to crash since it can't obtain
a name for a device, when the name is occupied by another device:

#!/bin/bash
ifconfig eth0 down
ifconfig eth1 down
index=`cat /sys/class/net/eth1/ifindex`
ip link set eth1 name dev$index
unshare -n sleep 1h &
pid=$!
while [[ "`readlink /proc/self/ns/net`" == "`readlink /proc/$pid/ns/net`" ]]; do continue; done
ip link set dev$index netns $pid
ip link set eth0 name dev$index
kill -9 $pid

Kernel messages:

virtio_net virtio1 dev3: renamed from eth1
virtio_net virtio0 dev3: renamed from eth0
default_device_exit: failed to move dev3 to init_net: -17
------------[ cut here ]------------
kernel BUG at net/core/dev.c:8978!
invalid opcode: 0000 [#1] PREEMPT SMP
CPU: 1 PID: 276 Comm: kworker/u8:3 Not tainted 4.17.0+ torvalds#292
Workqueue: netns cleanup_net
RIP: 0010:default_device_exit+0x9c/0xb0
[stack trace snipped]

This patch gives more variability during choosing new name
of device and fixes the problem.

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>

net/core/dev.c

@@ -8959,7 +8959,6 @@ static void __net_exit default_device_exit(struct net *net)
 	rtnl_lock();
 	for_each_netdev_safe(net, dev, aux) {
 		int err;
-		char fb_name[IFNAMSIZ];
 
 		/* Ignore unmoveable devices (i.e. loopback) */
 		if (dev->features & NETIF_F_NETNS_LOCAL)
@@ -8970,8 +8969,7 @@ static void __net_exit default_device_exit(struct net *net)
 			continue;
 
 		/* Push remaining network devices to init_net */
-		snprintf(fb_name, IFNAMSIZ, "dev%d", dev->ifindex);
-		err = dev_change_net_namespace(dev, &init_net, fb_name);
+		err = dev_change_net_namespace(dev, &init_net, "dev%d");
 		if (err) {
 			pr_emerg("%s: failed to move %s to init_net: %d\n",
 				 __func__, dev->name, err);