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.