io_uring: ensure ->list is initialized for poll commands

Daniel reports that when testing an http server that uses io_uring
to poll for incoming connections, sometimes it hard crashes. This is
due to an uninitialized list member for the io_uring request. Normally
this doesn't trigger and none of the test cases caught it.
Reported-by: Daniel Kozak <kozzi11@gmail.com>
Tested-by: Daniel Kozak <kozzi11@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1666,6 +1666,8 @@ static int io_poll_add(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	INIT_LIST_HEAD(&poll->wait.entry);
 	init_waitqueue_func_entry(&poll->wait, io_poll_wake);
 
+	INIT_LIST_HEAD(&req->list);
+
 	mask = vfs_poll(poll->file, &ipt.pt) & poll->events;
 
 	spin_lock_irq(&ctx->completion_lock);