mm: define PAGE_MAPPING_FLAGS

At present we define PageAnon(page) by the low PAGE_MAPPING_ANON bit set
in page->mapping, with the higher bits a pointer to the anon_vma; and have
defined PageKsm(page) as that with NULL anon_vma.

But KSM swapping will need to store a pointer there: so in preparation for
that, now define PAGE_MAPPING_FLAGS as the low two bits, including
PAGE_MAPPING_KSM (always set along with PAGE_MAPPING_ANON, until some
other use for the bit emerges).

Declare page_rmapping(page) to return the pointer part of page->mapping,
and page_anon_vma(page) to return the anon_vma pointer when that's what it
is.  Use these in a few appropriate places: notably, unuse_vma() has been
testing page->mapping, but is better to be testing page_anon_vma() (cases
may be added in which flag bits are set without any pointer).

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>