mm: clean up mm_counter

Presently, per-mm statistics counter is defined by macro in sched.h

This patch modifies it to
  - defined in mm.h as inlinf functions
  - use array instead of macro's name creation.

This patch is for reducing patch size in future patch to modify
implementation of per-mm counter.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/filemap_xip.c b/mm/filemap_xip.c
index 1888b2d..78b94f0 100644
--- a/mm/filemap_xip.c
+++ b/mm/filemap_xip.c
@@ -194,7 +194,7 @@
 			flush_cache_page(vma, address, pte_pfn(*pte));
 			pteval = ptep_clear_flush_notify(vma, address, pte);
 			page_remove_rmap(page);
-			dec_mm_counter(mm, file_rss);
+			dec_mm_counter(mm, MM_FILEPAGES);
 			BUG_ON(pte_dirty(pteval));
 			pte_unmap_unlock(pte, ptl);
 			page_cache_release(page);