io_uring: ensure req->submit is copied when req is deferred

There's an issue with deferred requests through drain, where if we do
need to defer, we're not copying over the sqe_submit state correctly.
This can result in using uninitialized data when we then later go and
submit the deferred request, like this check in __io_submit_sqe():

         if (unlikely(s->index >= ctx->sq_entries))
                 return -EINVAL;

with 's' being uninitialized, we can randomly fail this check. Fix this
by copying sqe_submit state when we defer a request.

Because it was fixed as part of a cleanup series in mainline, before
anyone realized we had this issue. That removed the separate states
of ->index vs ->submit.sqe. That series is not something I was
comfortable putting into stable, hence the much simpler addition.
Here's the patch in the series that fixes the same issue:

commit cf6fd4bd
Author: Pavel Begunkov <asml.silence@gmail.com>
Date:   Mon Nov 25 23:14:39 2019 +0300

    io_uring: inline struct sqe_submit

Reported-by: Andres Freund <andres@anarazel.de>
Reported-by: Tomáš Chaloupka
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>