Make vm statistics update interval configurable

Make it configurable.  Code in mm makes the vm statistics intervals
independent from the cache reaper use that opportunity to make it
configurable.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>