Input: evdev - only allow reading events if a full packet is present

Without this, it was possible for the reader to get ahead of packet_head.
If the input device generated a partial packet *right* after the reader
got ahead, then we can get into a situation where the device is marked
readable, but read always returns 0 until the next packet is finished
(i.e a SYN is generated by the input driver).

This situation can also happen if we overflow the buffer while a reader
is trying to read an event out.
Signed-off-by: Dima Zavin <dima@android.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

drivers/input/evdev.c

@@ -369,7 +369,7 @@ static int evdev_fetch_next_event(struct evdev_client *client,
 
 	spin_lock_irq(&client->buffer_lock);
 
-	have_event = client->head != client->tail;
+	have_event = client->packet_head != client->tail;
 	if (have_event) {
 		*event = client->buffer[client->tail++];
 		client->tail &= client->bufsize - 1;