[$] 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 bugs
in the kernel. Protecting against this possibility is the objective of this
patch set from Ira Weiny, which makes use of Intel's 'protection keys
supervisor' (PKS) feature to make it harder for the kernel to inadvertently write
to persistent memory.