io_uring: Fix race condition when sqp thread goes to sleep

If an asynchronous completion happens before the task is preparing
itself to wait and set its state to TASK_INTERRUPTIBLE, the completion
will not wake up the sqp thread.

Cc: stable@vger.kernel.org
Signed-off-by: Olivier Langlois <olivier@trillion01.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/d1419dc32ec6a97b453bee34dc03fa6a02797142.1624473200.git.olivier@trillion01.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>