md/raid10: don't modify 'nr_waitng' in wait_barrier() for the case nowait

For the case nowait in wait_barrier(), there is no point to increase
nr_waiting and then decrease it.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Acked-by: Guoqing Jiang <guoqing.jiang@linux.dev>
Signed-off-by: Song Liu <song@kernel.org>