handle_stripe_expansion contains:

        if (tx) {
                async_tx_ack(tx);
                dma_wait_for_async_tx(tx);
        }

which is very similar to the body of async_tx_quiesce(),
except that the later handles an error from dma_wait_for_async_tx()
(admittedly by panicing, but that decision belongs in the dma
code, not the md code).

So just us async_tx_quiesce().
Acked-by: NDan Williams <djbw@fb.com>
Reported-by: NBartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: NNeilBrown <neilb@suse.de>