net-sysfs: Use rtnl_trylock in sysfs methods.

The earlier patch to fix the deadlock between a network device going
away and writing to sysfs attributes was incomplete.
- It did not set signal_pending so we would leak ERSTARTSYS to user space.
- It used ERESTARTSYS which only restarts if sigaction configures it to.
- It did not cover store and show for ifalias.

So fix all of these up and use the new helper restart_syscall so we get
the details correct on what it takes.
Signed-off-by: NEric W. Biederman <ebiederm@aristanetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/net-sysfs.c

@@ -78,7 +78,7 @@ static ssize_t netdev_store(struct device *dev, struct device_attribute *attr,
 		goto err;
 
 	if (!rtnl_trylock())
-		return -ERESTARTSYS;
+		return restart_syscall();
 
 	if (dev_isalive(net)) {
 		if ((ret = (*set)(net, new)) == 0)
@@ -225,7 +225,8 @@ static ssize_t store_ifalias(struct device *dev, struct device_attribute *attr,
 	if (len >  0 && buf[len - 1] == '\n')
 		--count;
 
-	rtnl_lock();
+	if (!rtnl_trylock())
+		return restart_syscall();
 	ret = dev_set_alias(netdev, buf, count);
 	rtnl_unlock();
 
@@ -238,7 +239,8 @@ static ssize_t show_ifalias(struct device *dev,
 	const struct net_device *netdev = to_net_dev(dev);
 	ssize_t ret = 0;
 
-	rtnl_lock();
+	if (!rtnl_trylock())
+		return restart_syscall();
 	if (netdev->ifalias)
 		ret = sprintf(buf, "%s\n", netdev->ifalias);
 	rtnl_unlock();