writeback: drop now-unnecessary rcu_barrier() in cgroup_writeback_umount()

writeback: drop now-unnecessary rcu_barrier() in cgroup_writeback_umount()

Commit e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when
switching inodes") replaced the queue_rcu_work() based scheduling of
inode wb switches with a plain queue_work().  Since then no switcher
goes through call_rcu(), so rcu_barrier() in cgroup_writeback_umount()
has no callbacks of its own to wait for.  It still drains unrelated
call_rcu() callbacks from other subsystems on busy systems, which
incidentally slows umount down; drop it.

Fixes: e1b849cfa6b6 ("writeback: Avoid contention on wb->list_lock when switching inodes")
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Baokun Li <libaokun@linux.alibaba.com>
Link: https://patch.msgid.link/20260521095016.2791354-3-libaokun@linux.alibaba.com
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>