Change pagemap output format to allow for future reporting of huge pages

Change pagemap output format to allow for future reporting of huge pages.

(Format comment and minor cleanups: mpm@selenic.com)

Signed-off-by: Hans Rosenfeld <hans.rosenfeld@amd.com>
Signed-off-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>