io_uring: simplify do_read return parsing

do_read() returning 0 bytes read (not -EAGAIN/etc.) is not an important
enough of a case to prioritise it. Fold it into ret < 0 check, so we get
rid of an extra if and make it a bit more readable.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>