HID: fix waitqueue usage in hiddev

DECLARE_WAITQUEUE doesn't initialize the wait descriptor's task_list
to 'empty' but to zero.

prepare_to_wait() will not enqueue the descriptor to the waitqueue and
finish_wait() will do list_del_init() on a list head that contains
NULL pointers, which oopses.

This was introduced by 0790340 "HID: hiddev cleanup -- handle all
error conditions properly".

The prior code used an unconditional add_to_waitqueue() which didn't
care about the wait descriptor's list head and enqueued the thing
unconditionally.

The new code uses prepare_to_wait() which DOES check the prior list
state, so use DEFINE_WAIT instead.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Oliver Neukum <oliver@neukum.name>
Cc: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>

drivers/hid/usbhid/hiddev.c

@@ -323,7 +323,7 @@ static ssize_t hiddev_write(struct file * file, const char __user * buffer, size
  */
 static ssize_t hiddev_read(struct file * file, char __user * buffer, size_t count, loff_t *ppos)
 {
-	DECLARE_WAITQUEUE(wait, current);
+	DEFINE_WAIT(wait);
 	struct hiddev_list *list = file->private_data;
 	int event_size;
 	int retval;