cgroup: rstat: call cgroup_rstat_updated_list with cgroup_rstat_lock

The commit 093c881 ("cgroup: rstat: Cleanup flushing functions and
locking") during cleanup accidentally changed the code to call
cgroup_rstat_updated_list() without cgroup_rstat_lock which is required.
Fix it.

Fixes: 093c881 ("cgroup: rstat: Cleanup flushing functions and locking")
Reported-by: Jakub Kicinski <kuba@kernel.org>
Reported-by: Breno Leitao <leitao@debian.org>
Reported-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Closes: https://lore.kernel.org/all/6564c3d6-9372-4352-9847-1eb3aea07ca4@linux.ibm.com/
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/rstat.c

@@ -318,10 +318,11 @@ __bpf_kfunc void cgroup_rstat_flush(struct cgroup *cgrp)
 
 	might_sleep();
 	for_each_possible_cpu(cpu) {
-		struct cgroup *pos = cgroup_rstat_updated_list(cgrp, cpu);
+		struct cgroup *pos;
 
 		/* Reacquire for each CPU to avoid disabling IRQs too long */
 		__cgroup_rstat_lock(cgrp, cpu);
+		pos = cgroup_rstat_updated_list(cgrp, cpu);
 		for (; pos; pos = pos->rstat_flush_next) {
 			struct cgroup_subsys_state *css;