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 <vapier@gentoo.org>
Cc: David Howells <dhowells@redhat.com>
Acked-by: David McCullough <davidm@snapgear.com>
Cc: Greg Ungerer <gerg@uclinux.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>