mm/ksm: add pages scanned metric

ksm currently maintains several statistics, which let you determine how
successful KSM is at sharing pages.  However it does not contain a metric
to determine how much work it does.

This commit adds the pages scanned metric.  This allows the administrator
to determine how many pages have been scanned over a period of time.

Link: https://lkml.kernel.org/r/20230811193655.2518943-1-shr@devkernel.io


Signed-off-by: Stefan Roesch <shr@devkernel.io>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Rik van Riel <riel@surriel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>