mm: fix rss count leakage during migration

Memory migration fills a pte with a migration entry and it doesn't
update the rss counters.  Then it replaces the migration entry with the
new page (or the old one if migration failed).  But between these two
passes this pte can be unmaped, or a task can fork a child and it will
get a copy of this migration entry.  Nobody accounts for this in the rss
counters.

This patch properly adjust rss counters for migration entries in
zap_pte_range() and copy_one_pte().  Thus we avoid extra atomic
operations on the migration fast-path.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>