ring-buffer: Add checking nr_subbufs to persistent ring buffer validation

In the Linux kernel, the following vulnerability has been resolved: ring-buffer: Add checking nr_subbufs to persistent ring buffer validation Sashiko reported that the code was using meta->nr_subbufs without making sure that it matched the nr_pages + 1 on data that was assuming the two were the same. Add a check to the persistent ring buffer validation code to make sure that the saved nr_subbufs matches what we expect.

Where it's fixed