Btrfs: Use balance_dirty_pages_nr on btree blocks

btrfs_btree_balance_dirty is changed to pass the number of pages dirtied
for more accurate dirty throttling.  This lets the VM make better decisions
about when to force some writeback.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 26826a5..698eaea 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -655,7 +655,7 @@
 		num_written += write_bytes;
 
 		balance_dirty_pages_ratelimited_nr(inode->i_mapping, num_pages);
-		btrfs_btree_balance_dirty(root);
+		btrfs_btree_balance_dirty(root, 1);
 		cond_resched();
 	}
 	mutex_unlock(&inode->i_mutex);