io_uring: verify pad field is 0 in io_get_ext_arg

io_uring: verify pad field is 0 in io_get_ext_arg

BugLink: https://bugs.launchpad.net/bugs/1969857
[ Upstream commit d2347b9695dafe5c388a5f9aeb70e27a7a4d29cf ]

Ensure that only 0 is passed for pad here.

Fixes: c73ebb685fb6 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>