writeback: add /sys/devices/system/node/<node>/vmstat

For NUMA node systems it is important to have visibility in memory
characteristics.  Two of the /proc/vmstat values "nr_written" and
"nr_dirtied" are added here.

	# cat /sys/devices/system/node/node20/vmstat
	nr_written 0
	nr_dirtied 0

Signed-off-by: Michael Rubin <mrubin@google.com>
Reviewed-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>