pagemap: return map count, not reference count, in /proc/kpagecount

Since pagemap is all about examining pages mapped into processes' memory
spaces, it makes sense for kpagecount to return the map counts, not the
reference counts.

Signed-off-by: Thomas Tuttle <ttuttle@google.com>
Cc: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>