md/raid10: fix null ptr dereference in raid10_size()

In raid10_run() if raid10_set_queue_limits() succeed, the return value
is set to zero, and if following procedures failed raid10_run() will
return zero while mddev->private is still NULL, causing null ptr
dereference in raid10_size().

Fix the problem by only overwrite the return value if
raid10_set_queue_limits() failed.

Fixes: 3d8466ba ("md/raid10: use the atomic queue limit update APIs")
Cc: stable@vger.kernel.org
Reported-and-tested-by: ValdikSS <iam@valdikss.org.ru>
Closes: https://lore.kernel.org/all/0dd96820-fe52-4841-bc58-dbf14d6bfcc8@valdikss.org.ru/


Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20241009014914.1682037-1-yukuai1@huaweicloud.com


Signed-off-by: Song Liu <song@kernel.org>