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>

fs/btrfs/extent-tree.c

@@ -1275,6 +1275,7 @@ struct buffer_head *btrfs_alloc_free_block(struct btrfs_trans_handle *trans,
 	set_buffer_checked(buf);
 	set_buffer_defrag(buf);
 	set_radix_bit(&trans->transaction->dirty_pages, buf->b_page->index);
+	trans->blocks_used++;
 	return buf;
 }