clear_refs: redefine callback functions for page table walker

clear_refs: redefine callback functions for page table walker

Currently clear_refs_pte_range() is connected to pmd_entry() to split thps
if found.  But now this work can be done in core page table walker code.
So we have no reason to keep this callback on pmd_entry().  This patch
moves pte handling code on pte_entry() callback.

clear_refs_write() has some prechecks about if we really walk over a given
vma.  It's fine to let them done by test_walk() callback, so let's define
it.

Signed-off-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Matt Mackall <mpm@selenic.com>
Cc: Cliff Wickman <cpw@sgi.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>