io_uring: ensure poll commands clear ->sqe

If we end up getting woken in poll (due to a signal), then we may need
to punt the poll request to an async worker. When we do that, we look up
the list to queue at, deferefencing req->submit.sqe, however that is
only set for requests we initially decided to queue async.

This fixes a crash with poll command usage and wakeups that need to punt
to async context.

Fixes: 54a91f3b ("io_uring: limit parallelism of buffered writes")
Signed-off-by: Jens Axboe <axboe@kernel.dk>