cgroup: fix uninitialised iterator in for_each_subsys_which

Fix the fact that @ssid is uninitialised in the case where
CGROUP_SUBSYS_COUNT = 0 by setting ssid to 0.

Fixes: cb4a31675270 ("cgroup: use bitmask to filter for_each_subsys")
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -424,7 +424,7 @@ static int notify_on_release(const struct cgroup *cgrp)
  */
 #define for_each_subsys_which(ss, ssid, ss_maskp)			\
 	if (!CGROUP_SUBSYS_COUNT) /* to avoid spurious gcc warning */	\
-		;							\
+		(ssid) = 0;						\
 	else								\
 		for_each_set_bit(ssid, ss_maskp, CGROUP_SUBSYS_COUNT)	\
 			if (((ss) = cgroup_subsys[ssid]) && false)	\