[$] Stray-write protection for persistent memory

Persistent memory has a number of advantages; it is fast, CPU-addressable,available in large quantities and, of course, persistent. But it also,arguably, poses a higher risk of suffering corruption as a result of bugsin the kernel. Protecting against this possibility is the objective of thispatch set from Ira Weiny, which makes use of Intel's "protection keyssupervisor" (PKS) feature to make it harder for the kernel to inadvertently writeto persistent memory.