mm: introduce wrappers to access mm->nr_ptes

Let's add wrappers for ->nr_ptes with the same interface as for nr_pmd
and nr_pud.

The patch also makes nr_ptes accounting dependent onto CONFIG_MMU.  Page
table accounting doesn't make sense if you don't have page tables.

It's preparation for consolidation of page-table counters in mm_struct.

Link: http://lkml.kernel.org/r/20171006100651.44742-1-kirill.shutemov@linux.intel.com
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/khugepaged.c

diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 43cb304..ea4ff25 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -1270,7 +1270,7 @@ static void retract_page_tables(struct address_space *mapping, pgoff_t pgoff)
 			_pmd = pmdp_collapse_flush(vma, addr, pmd);
 			spin_unlock(ptl);
 			up_write(&vma->vm_mm->mmap_sem);
-			atomic_long_dec(&vma->vm_mm->nr_ptes);
+			mm_dec_nr_ptes(vma->vm_mm);
 			pte_free(vma->vm_mm, pmd_pgtable(_pmd));
 		}
 	}