KSM was converted to use rmap_walk() and now nobody uses these functions
outside mm/rmap.c.

Let's covert them back to static.
Signed-off-by: NKirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>