char: nwbutton: open-code interruptible_sleep_on

The nwbutton driver uses interruptible_sleep_on to wait for buttons
getting pressed after we enter the read() function, which is inherently
racy and cannot be fixed by using wait_event without changing the
driver's user space interface.

Instead, this patch just uses an open-coded variant of the same
interruptible_sleep_on() call, so the driver behavior doesn't change
but we remove the sleep_on family from the kernel.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/char/nwbutton.c

@@ -168,7 +168,10 @@ static irqreturn_t button_handler (int irq, void *dev_id)
 static int button_read (struct file *filp, char __user *buffer,
 			size_t count, loff_t *ppos)
 {
-	interruptible_sleep_on (&button_wait_queue);
+	DEFINE_WAIT(wait);
+	prepare_to_wait(&button_wait_queue, &wait, TASK_INTERRUPTIBLE);
+	schedule();
+	finish_wait(&button_wait_queue, &wait);
 	return (copy_to_user (buffer, &button_output_buffer, bcount))
 		 ? -EFAULT : bcount;
 }