mm: count reclaimable pages per BDI

Count per BDI reclaimable pages; nr_reclaimable = nr_dirty + nr_unstable.

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>
diff --git a/fs/buffer.c b/fs/buffer.c
index faceb5e..86e58b1 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -710,6 +710,8 @@
 
 		if (mapping_cap_account_dirty(mapping)) {
 			__inc_zone_page_state(page, NR_FILE_DIRTY);
+			__inc_bdi_stat(mapping->backing_dev_info,
+					BDI_RECLAIMABLE);
 			task_io_account_write(PAGE_CACHE_SIZE);
 		}
 		radix_tree_tag_set(&mapping->page_tree,