mm,vmacache: add debug data

mm,vmacache: add debug data

Introduce a CONFIG_DEBUG_VM_VMACACHE option to enable counting the cache
hit rate -- exported in /proc/vmstat.

Any updates to the caching scheme needs this kind of data, thus it can
save some work re-implementing the counting all the time.

Signed-off-by: Davidlohr Bueso <[email protected]>
Cc: Aswin Chandramouleeswaran <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>