Input: evdev - do not block waiting for an event if fd is nonblock

If there is a full packet in the buffer, and we overflow that buffer
right after checking for that condition, it would have been possible
for us to block indefinitely (rather, until the next full packet) even if
the file was marked as O_NONBLOCK.

Cc: Jeff Brown <jeffbrown@android.com>
Signed-off-by: Dima Zavin <dima@android.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/evdev.c

@@ -391,14 +391,13 @@ static ssize_t evdev_read(struct file *file, char __user *buffer,
 	if (count < input_event_size())
 		return -EINVAL;
 
-	if (client->packet_head == client->tail && evdev->exist &&
-	    (file->f_flags & O_NONBLOCK))
-		return -EAGAIN;
-
-	retval = wait_event_interruptible(evdev->wait,
-		client->packet_head != client->tail || !evdev->exist);
-	if (retval)
-		return retval;
+	if (!(file->f_flags & O_NONBLOCK)) {
+		retval = wait_event_interruptible(evdev->wait,
+				client->packet_head != client->tail ||
+				!evdev->exist);
+		if (retval)
+			return retval;
+	}
 
 	if (!evdev->exist)
 		return -ENODEV;