cgroup: Avoid -Wstringop-overflow warnings

Address the following -Wstringop-overflow warnings seen when
built with ARM architecture and aspeed_g4_defconfig configuration
(notice that under this configuration CGROUP_SUBSYS_COUNT == 0):
kernel/cgroup/cgroup.c:1208:16: warning: 'find_existing_css_set' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
kernel/cgroup/cgroup.c:1258:15: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
kernel/cgroup/cgroup.c:6089:18: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]
kernel/cgroup/cgroup.c:6153:18: warning: 'css_set_hash' accessing 4 bytes in a region of size 0 [-Wstringop-overflow=]

These changes are based on commit d20d30eb ("cgroup: Avoid compiler
warnings with no subsystems").
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup.c

@@ -1198,6 +1198,9 @@ static struct css_set *find_css_set(struct css_set *old_cset,
 	unsigned long key;
 	int ssid;
 
+	if (!CGROUP_HAS_SUBSYS_CONFIG)
+		return NULL;
+
 	lockdep_assert_held(&cgroup_mutex);
 
 	/* First see if we already have a cgroup group that matches
@@ -6017,6 +6020,9 @@ int __init cgroup_init(void)
 	struct cgroup_subsys *ss;
 	int ssid;
 
+	if (!CGROUP_HAS_SUBSYS_CONFIG)
+		return -EINVAL;
+
 	BUILD_BUG_ON(CGROUP_SUBSYS_COUNT > 16);
 	BUG_ON(cgroup_init_cftypes(NULL, cgroup_base_files));
 	BUG_ON(cgroup_init_cftypes(NULL, cgroup_psi_files));