ksm: make rmap walks more scalable

The rmap walks in ksm.c are like those in rmap.c: they can safely be
done with anon_vma_lock_read().

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>