rmap: fixup page_referenced() for nommu systems

After the recent changes that went into mm/vmscan.c to overhaul stuff, we
ended up with these warnings on no-mmu systems:

mm/vmscan.c: In function `shrink_page_list':
mm/vmscan.c:580: warning: unused variable `vm_flags'
mm/vmscan.c: In function `shrink_active_list':
mm/vmscan.c:1294: warning: `vm_flags' may be used uninitialized in this function
mm/vmscan.c:1242: note: `vm_flags' was declared here

This is because the no-mmu function defines page_referenced() to work on
the first argument only (the page). It does not clear the vm_flags given
to it because for no-mmu systems, they never actually get utilized. Since
that is no longer strictly true, we need to set vm_flags to 0 like
everyone else so gcc can do proper dead code elimination without annoying
us with unused warnings.

Signed-off-by: Mike Frysinger <[e-mail]>
Cc: David Howells <[e-mail]>
Acked-by: David McCullough <[e-mail]>
Cc: Greg Ungerer <[e-mail]>
Cc: Paul Mundt <[e-mail]>
Cc: Wu Fengguang <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/rmap.h

···
 #define anon_vma_prepare(vma)	(0)
 #define anon_vma_link(vma)	do {} while (0)
 
-#define page_referenced(page, locked, cnt, flags) TestClearPageReferenced(page)
+static inline int page_referenced(struct page *page, int is_locked,
+				  struct mem_cgroup *cnt,
+				  unsigned long *vm_flags)
+{
+	*vm_flags = 0;
+	return TestClearPageReferenced(page);
+}
+
 #define try_to_unmap(page, refs) SWAP_FAIL
 
 static inline int page_mkclean(struct page *page)