writeback: clear old wb dirty state after inode switches

Switching an inode to another wb updates the target wb's dirty-I/O state,
but no longer checks whether the source wb's dirty-I/O lists became empty.
The check was lost when inode_io_list_del_locked(inode, old_wb) was
removed to support tracking clean attached inodes.

If the last dirty inode is moved, WB_has_dirty_io remains set on the old
wb and its bandwidth contribution remains in bdi->tot_write_bandwidth.
This can keep an empty wb scheduled for periodic writeback and overstate
the total bandwidth used to distribute writeback work.

Call wb_io_lists_depopulated() on the old wb after the batch of inode
switches, while its list_lock is still held. This clears the flag and
removes the bandwidth contribution if its dirty-I/O lists are empty.

Fixes: f3b6a6df38 ("writeback, cgroup: keep list of inodes attached to bdi_writeback")
Cc: stable@vger.kernel.org
Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
Link: https://patch.msgid.link/20261016115230.3640770-3-sunjunchao@bytedance.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

fs/fs-writeback.c

@@ -566,6 +566,7 @@ static void process_inode_switch_wbs(struct bdi_writeback *new_wb,
 		}
 	}
 
+	wb_io_lists_depopulated(old_wb);
 	spin_unlock(&new_wb->list_lock);
 	spin_unlock(&old_wb->list_lock);