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

In the Linux kernel, the following vulnerability has been resolved: 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.