Btrfs: only clear dirty on the buffer if it is marked as dirty

No reason to set the path blocking or loop through all of the pages if the
extent buffer isn't actually marked dirty.  Thanks,
Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/disk-io.c

@@ -1142,12 +1142,12 @@ void clean_tree_block(struct btrfs_trans_handle *trans, struct btrfs_root *root,
 					  root->fs_info->dirty_metadata_bytes);
 			}
 			spin_unlock(&root->fs_info->delalloc_lock);
-		}
 
 			/* ugh, clear_extent_buffer_dirty needs to lock the page */
 			btrfs_set_lock_blocking(buf);
 			clear_extent_buffer_dirty(buf);
 		}
+	}
 }
 
 static void __setup_root(u32 nodesize, u32 leafsize, u32 sectorsize,