netdevsim: switch to memdup_user_nul()

Use memdup_user_nul() helper instead of open-coding to
simplify the code.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/netdevsim/health.c

@@ -235,15 +235,10 @@ static ssize_t nsim_dev_health_break_write(struct file *file,
 	char *break_msg;
 	int err;
 
-	break_msg = kmalloc(count + 1, GFP_KERNEL);
-	if (!break_msg)
-		return -ENOMEM;
+	break_msg = memdup_user_nul(data, count);
+	if (IS_ERR(break_msg))
+		return PTR_ERR(break_msg);
 
-	if (copy_from_user(break_msg, data, count)) {
-		err = -EFAULT;
-		goto out;
-	}
-	break_msg[count] = '\0';
 	if (break_msg[count - 1] == '\n')
 		break_msg[count - 1] = '\0';