io_uring: fix NULL mm in io_poll_task_func()

io_poll_task_func() hand-coded link submission forgetting to set
TASK_RUNNING, acquire mm, etc. Call existing helper for that,
i.e. __io_req_task_submit().

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>