CVE-2025-38584: Use After Free

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

padata: Fix pd UAF once and for all

There is a race condition/UAF in padata_reorder that goes back to the initial commit. A reference count is taken at the start of the process in padata_do_parallel, and released at the end in padata_serial_worker.

This reference count is (and only is) required for padata_replace to function correctly. If padata_replace is never called then there is no issue.

In the function padata_reorder which serves as the core of padata, as soon as padata is added to queue->serial.list, and the associated spin lock released, that padata may be processed and the reference count on pd would go away.

Fix this by getting the next padata before the squeue->serial lock is released.

In order to make this possible, simplify padata_reorder by only calling it once the next padata arrives.