CVE-2024-46713: Out-of-bounds Write

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

perf/aux: Fix AUX buffer serialization

Ole reported that event->mmap_mutex is strictly insufficient to serialize the AUX buffer, add a per RB mutex to fully serialize it.

Note that in the lock order comment the perf_event::mmap_mutex order was already wrong, that is, it nesting under mmap_lock is not new with this patch.