cgroup: CGRP_ROOT_SUBSYS_BOUND should also be ignored when mounting an existing hierarchy

0ce6cba357 ("cgroup: CGRP_ROOT_SUBSYS_BOUND should be ignored when
comparing mount options") only updated the remount path but
CGRP_ROOT_SUBSYS_BOUND should also be ignored when comparing options
while mounting an existing hierarchy.  As option mismatch triggers a
warning but doesn't fail the mount without sane_behavior, this only
triggers a spurious warning message.

Fix it by only comparing CGRP_ROOT_OPTION_MASK bits when comparing new
and existing root options.

Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -1703,7 +1703,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 		 */
 		cgroup_free_root(opts.new_root);
 
-		if (root->flags != opts.flags) {
+		if ((root->flags ^ opts.flags) & CGRP_ROOT_OPTION_MASK) {
 			if ((root->flags | opts.flags) & CGRP_ROOT_SANE_BEHAVIOR) {
 				pr_err("cgroup: sane_behavior: new mount options should match the existing superblock\n");
 				ret = -EINVAL;