mm/rmap: extend rmap_walk_xxx() to cope with different cases

There are a lot of common parts in traversing functions, but there are
also a little of uncommon parts in it.  By assigning proper function
pointer on each rmap_walker_control, we can handle these difference
correctly.

Following are differences we should handle.

1. difference of lock function in anon mapping case
2. nonlinear handling in file mapping case
3. prechecked condition:
	checking memcg in page_referenced(),
	checking VM_SHARE in page_mkclean()
	checking temporary vma in try_to_unmap()
4. exit condition:
	checking page_mapped() in try_to_unmap()

So, in this patch, I introduce 4 function pointers to handle above
differences.

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Hillf Danton <dhillf@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>