CVE-2026-64318 partitions: aix: bound the pp_count scan to the ppe array

In the Linux kernel, the following vulnerability has been resolved: partitions: aix: bound the ppcount scan to the ppe array aixpartition reads the physical volume descriptor into a fixed-size struct pvd and then scans its physical-partition-extent array: int numpps = be16tocpupvd-ppcount; ... fo...