io_uring: verify pad field is 0 in io_get_ext_arg

Ensure that only 0 is passed for pad here.

Fixes: c73ebb68

 ("io_uring: add timeout support for io_uring_enter()")
Signed-off-by: Dylan Yudaken <dylany@fb.com>
Link: https://lore.kernel.org/r/20220412163042.2788062-5-dylany@fb.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>