mm: Add PAGE_ALIGN_DOWN macro

This is just the same as PAGE_ALIGN(), but rounds the address down, not
up.
Suggested-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: David Gow <davidgow@google.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Richard Weinberger <richard@nod.at>

include/linux/mm.h

@@ -221,6 +221,9 @@ int overcommit_policy_handler(struct ctl_table *, int, void *, size_t *,
 /* to align the pointer to the (next) page boundary */
 #define PAGE_ALIGN(addr) ALIGN(addr, PAGE_SIZE)
 
+/* to align the pointer to the (prev) page boundary */
+#define PAGE_ALIGN_DOWN(addr) ALIGN_DOWN(addr, PAGE_SIZE)
+
 /* test whether an address (unsigned long or pointer) is aligned to PAGE_SIZE */
 #define PAGE_ALIGNED(addr)	IS_ALIGNED((unsigned long)(addr), PAGE_SIZE)