mm: modify swap_map and add SWAP_HAS_CACHE flag

This is a part of the patches for fixing memcg's swap accountinf leak.
But, IMHO, not a bad patch even if no memcg.

There are 2 kinds of references to swap.
 - reference from swap entry
 - reference from swap cache

Then,

 - If there is swap cache && swap's refcnt is 1, there is only swap cache.
  (*) swapcount(entry) == 1 && find_get_page(swapper_space, entry) != NULL

This counting logic have worked well for a long time.  But considering
that we cannot know there is a _real_ reference or not by swap_map[],
current usage of counter is not very good.

This patch adds a flag SWAP_HAS_CACHE and recored information that a swap
entry has a cache or not.  This will remove -1 magic used in swapfile.c
and be a help to avoid unnecessary find_get_page().

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Tested-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>