io_uring: fix ctx refcounting in io_submit_sqes()

If io_get_req() fails, it drops a ref. Then, awhile keeping @submitted
unmodified, io_submit_sqes() breaks the loop and puts @nr - @submitted
refs. For each submitted req a ref is dropped in io_put_req() and
friends. So, for @nr taken refs there will be
(@nr - @submitted + @submitted + 1) dropped.

Remove ctx refcounting from io_get_req(), that at the same time makes
it clearer.

Fixes: 2b85edfc ("io_uring: batch getting pcpu references")
Cc: stable@vger.kernel.org # v5.6
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1342,7 +1342,6 @@ static struct io_kiocb *io_get_req(struct io_ring_ctx *ctx,
 	req = io_get_fallback_req(ctx);
 	if (req)
 		goto got_it;
-	percpu_ref_put(&ctx->refs);
 	return NULL;
 }