convert a bunch of open-coded instances of memdup_user_nul()

A _lot_ of ->write() instances were open-coding it; some are
converted to memdup_user_nul(), a lot more remain...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/s390/char/vmcp.c

@@ -88,14 +88,9 @@ vmcp_write(struct file *file, const char __user *buff, size_t count,
 
 	if (count > 240)
 		return -EINVAL;
-	cmd = kmalloc(count + 1, GFP_KERNEL);
-	if (!cmd)
-		return -ENOMEM;
-	if (copy_from_user(cmd, buff, count)) {
-		kfree(cmd);
-		return -EFAULT;
-	}
-	cmd[count] = '\0';
+	cmd = memdup_user_nul(buff, count);
+	if (IS_ERR(cmd))
+		return PTR_ERR(cmd);
 	session = file->private_data;
 	if (mutex_lock_interruptible(&session->mutex)) {
 		kfree(cmd);