mm: introduce mm_find_pmd()

Several place need to find the pmd by(mm_struct, address), so introduce a
function to simplify it.

[akpm@linux-foundation.org: fix warning]
Signed-off-by: Bob Liu <lliubbo@gmail.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: Ni zhan Chen <nizhan.chen@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/internal.h b/mm/internal.h
index a4fa284..52d1fa9 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -92,6 +92,11 @@
 extern void putback_lru_page(struct page *page);
 
 /*
+ * in mm/rmap.c:
+ */
+extern pmd_t *mm_find_pmd(struct mm_struct *mm, unsigned long address);
+
+/*
  * in mm/page_alloc.c
  */
 extern void __free_pages_bootmem(struct page *page, unsigned int order);