mm: count writeback pages per BDI

Count per BDI writeback pages.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/backing-dev.h

@@ -28,6 +28,7 @@ typedef int (congested_fn)(void *, int);
 
 enum bdi_stat_item {
 	BDI_RECLAIMABLE,
+	BDI_WRITEBACK,
 	NR_BDI_STAT_ITEMS
 };