ksm: count allocated ksm rmap_items for each process

Patch series "ksm: count allocated rmap_items and update documentation",
v5.

KSM can save memory by merging identical pages, but also can consume
additional memory, because it needs to generate rmap_items to save each
scanned page's brief rmap information.

To determine how beneficial the ksm-policy (like madvise), they are using
brings, so we add a new interface /proc/<pid>/ksm_stat for each process
The value "ksm_rmap_items" in it indicates the total allocated ksm
rmap_items of this process.

The detailed description can be seen in the following patches' commit
message.


This patch (of 2):

KSM can save memory by merging identical pages, but also can consume
additional memory, because it needs to generate rmap_items to save each
scanned page's brief rmap information.  Some of these pages may be merged,
but some may not be abled to be merged after being checked several times,
which are unprofitable memory consumed.

The information about whether KSM ...