io_uring/rw: fix downgraded mshot read

The io-wq path can downgrade a multishot request to oneshot mode,
however io_read_mshot() doesn't handle that and would still post
multiple CQEs. That's not allowed, because io_req_post_cqe() requires
stricter context requirements.

The described can only happen with pollable files that don't support
FMODE_NOWAIT, which is an odd combination, so if even allowed it should
be fairly rare.

Cc: stable@vger.kernel.org
Reported-by: chase xd <sl1589472800@gmail.com>
Fixes: bee1d5be ("io_uring: disable io-wq execution of multishot NOWAIT requests")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/c5c8c4a50a882fd581257b81bf52eee260ac29fd.1735407848.git.asml.silence@gmail.com


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