cgroup: fix top cgroup refcnt leak

As mount() and kill_sb() is not a one-to-one match, If we mount the same
cgroupfs in serveral mount points, and then umount all of them, kill_sb()
will be called only once.

Try:
        # mount -t cgroup -o cpuacct xxx /cgroup
        # mount -t cgroup -o cpuacct xxx /cgroup2
        # cat /proc/cgroups | grep cpuacct
        cpuacct 2       1       1
        # umount /cgroup
        # umount /cgroup2
        # cat /proc/cgroups | grep cpuacct
        cpuacct 2       1       1

You'll see cgroupfs will never be freed.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -1487,6 +1487,7 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 	struct cgroup_sb_opts opts;
 	struct dentry *dentry;
 	int ret;
+	bool new_sb;
 
 	/*
 	 * The first time anyone tries to mount a cgroup, enable the list
@@ -1603,8 +1604,8 @@ static struct dentry *cgroup_mount(struct file_system_type *fs_type,
 	if (ret)
 		return ERR_PTR(ret);
 
-	dentry = kernfs_mount(fs_type, flags, root->kf_root, NULL);
-	if (IS_ERR(dentry))
+	dentry = kernfs_mount(fs_type, flags, root->kf_root, &new_sb);
+	if (IS_ERR(dentry) || !new_sb)
 		cgroup_put(&root->cgrp);
 	return dentry;
 }