io_uring: don't use poll handler if file can't be nonblocking read/written

io_uring: don't use poll handler if file can't be nonblocking read/written

[ Upstream commit 9dab14b81807a40dab8e464ec87043935c562c2c ]

There's no point in using the poll handler if we can't do a nonblocking
IO attempt of the operation, since we'll need to go async anyway. In
fact this is actively harmful, as reading from eg pipes won't return 0
to indicate EOF.

Cc: stable@vger.kernel.org # v5.7+
Reported-by: Benedikt Ames <wisp3rwind@posteo.eu>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>