io_uring: don't modify req->poll for rw

__io_queue_proc() is used by both poll and apoll, so we should not
access req->poll directly but selecting right struct io_poll_iocb
depending on use case.

Reported-and-tested-by:  <syzbot+a84b8783366ecb1c65d0@syzkaller.appspotmail.com>
Fixes: ea6a693d

 ("io_uring: disable multishot poll for double poll add cases")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/4a6a1de31142d8e0250fe2dfd4c8923d82a5bbfc.1621251795.git.asml.silence@gmail.com


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