writeback: account per-bdi accumulated written pages

Introduce the BDI_WRITTEN counter. It will be used for estimating the
bdi's write bandwidth.

Peter Zijlstra <[email protected]>:
Move BDI_WRITTEN accounting into __bdi_writeout_inc().
This will cover and fix fuse, which only calls bdi_writeout_inc().

CC: Michael Rubin <[email protected]>
Reviewed-by: KOSAKI Motohiro <[email protected]>
Signed-off-by: Jan Kara <[email protected]>
Signed-off-by: Wu Fengguang <[email protected]>

include/linux/backing-dev.h

@@ -40,6 +40,7 @@ typedef int (congested_fn)(void *, int);
 enum bdi_stat_item {
 	BDI_RECLAIMABLE,
 	BDI_WRITEBACK,
+	BDI_WRITTEN,
 	NR_BDI_STAT_ITEMS
 };