io_uring: fix sporadic double CQE entry for close

We punt close to async for the final fput(), but we log the completion
even before that even in that case. We rely on the request not having
a files table assigned to detect what the final async close should do.
However, if we punt the async queue to __io_queue_sqe(), we'll get
->files assigned and this makes io_close_finish() think it should both
close the filp again (which does no harm) AND log a new CQE event for
this request. This causes duplicate CQEs.

Queue the request up for async manually so we don't grab files
needlessly and trigger this condition.

Signed-off-by: Jens Axboe <axboe@kernel.dk>