fs: don't flush in-flight wb switches for superblocks without cgroup writeback

When deactivating any type of superblock, it had to wait for the in-flight
wb switches to be completed. wb switches are executed in inode_switch_wbs_work_fn()
which needs to acquire the wb_switch_rwsem and races against sync_inodes_sb().
If there are too much dirty data in the superblock, the waiting time may increase
significantly.

For superblocks without cgroup writeback such as tmpfs, they have nothing to
do with the wb swithes, so the flushing can be avoided.

Signed-off-by: Haifeng Xu <haifeng.xu@shopee.com>
Link: https://lore.kernel.org/r/20240726030525.180330-1-haifeng.xu@shopee.com


Reviewed-by: Jan Kara <jack@suse.cz>
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>