io_uring: tctx->task_lock should be IRQ safe

We add task_work from any context, hence we need to ensure that we can
tolerate it being from IRQ context as well.

Fixes: 7cbf1722

 ("io_uring: provide FIFO ordering for task_work")
Signed-off-by: Jens Axboe <axboe@kernel.dk>