io_uring/rw: fix cflags posting for single issue multishot read

If multishot gets disabled, and hence the request will get terminated
rather than persist for more iterations, then posting the CQE with the
right cflags is still important. Most notably, the buffer reference
needs to be included.

Refactor the return of __io_read() a bit, so that the provided buffer
is always put correctly, and hence returned to the application.

Reported-by: Sharon Rosner <Sharon Rosner>
Link: https://github.com/axboe/liburing/issues/1257


Cc: stable@vger.kernel.org
Fixes: 2a975d42 ("io_uring/rw: don't allow multishot reads without NOWAIT support")
Signed-off-by: Jens Axboe <axboe@kernel.dk>