cgroupfs: create /sys/fs/cgroup to mount cgroupfs on

We really shouldn't be asking userspace to create new root filesystems.
So follow along with all of the other in-kernel filesystems, and provide
a mount point in sysfs.

For cgroupfs, this should be in /sys/fs/cgroup/  This change provides
that mount point when the cgroup filesystem is registered in the kernel.
Acked-by: Paul Menage <menage@google.com>
Acked-by: Dhaval Giani <dhaval.giani@gmail.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Lennart Poettering <lennart@poettering.net>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

kernel/cgroup.c

@@ -1623,6 +1623,8 @@ static struct file_system_type cgroup_fs_type = {
 	.kill_sb = cgroup_kill_sb,
 };
 
+static struct kobject *cgroup_kobj;
+
 static inline struct cgroup *__d_cgrp(struct dentry *dentry)
 {
 	return dentry->d_fsdata;
@@ -3894,9 +3896,18 @@ int __init cgroup_init(void)
 	hhead = css_set_hash(init_css_set.subsys);
 	hlist_add_head(&init_css_set.hlist, hhead);
 	BUG_ON(!init_root_id(&rootnode));
+
+	cgroup_kobj = kobject_create_and_add("cgroup", fs_kobj);
+	if (!cgroup_kobj) {
+		err = -ENOMEM;
+		goto out;
+	}
+
 	err = register_filesystem(&cgroup_fs_type);
-	if (err < 0)
+	if (err < 0) {
+		kobject_put(cgroup_kobj);
 		goto out;
+	}
 
 	proc_create("cgroups", 0, NULL, &proc_cgroupstats_operations);