cgroup: use css_get() in cgroup_create() to check CSS_ROOT

cgroup: use css_get() in cgroup_create() to check CSS_ROOT

It seems that the root css doesn't have refcnt allocated(not needed?),
and would cause the booting error attached.

This patch tries to use css_get() to not increase the refcnt if parent
is root.

  BUG: unable to handle kernel NULL pointer dereference at           (null)
  IP: [<ffffffff810b37cc>] cgroup_mkdir+0x37c/0x740
  PGD 0
  Oops: 0002 [#1]
  Modules linked in:
  CPU: 0 PID: 1 Comm: systemd Not tainted 3.11.0-rc5-next-20130815+ #1
  Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
  task: ffff88007f868000 ti: ffff88007f864000 task.ti: ffff88007f864000
  RIP: 0010:[<ffffffff810b37cc>]  [<ffffffff810b37cc>] cgroup_mkdir+0x37c/0x740
  RSP: 0018:ffff88007f865df8  EFLAGS: 00010246
  RAX: 0000000000000000 RBX: ffffffff81a46ee0 RCX: 0000000000000001
  RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff81a415c0
  RBP: ffff88007f865ec8 R08: 0000000000000001 R09: 0000000000000000
  R10: ffff88007ce6d060 R11: 0000000000000000 R12: ffff88007ce6d000
  R13: ffff88007ce6d060 R14: ffffffff81a46d80 R15: ffff88007c6e8018
  FS:  00007f13dbf6f840(0000) GS:ffffffff81a23000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 0000000000000000 CR3: 000000007b7e5000 CR4: 00000000000006b0
  Stack:
   ffffffff810b380d 0000000000000002 ffff88007f865e18 ffffffff81167069
   ffff88007f865ed8 ffffffff8116a3f5 ffff880037454400 ffff88007c6e8018
   ffff88007c6e8028 ffff88007c6e8328 ffff88007c6e8000 ffff88007ce6d000
  Call Trace:
   [<ffffffff810b380d>] ? cgroup_mkdir+0x3bd/0x740
   [<ffffffff81167069>] ? lookup_hash+0x19/0x20
   [<ffffffff8116a3f5>] ? kern_path_create+0x95/0x170
   [<ffffffff8116ce3e>] vfs_mkdir+0x9e/0xf0
   [<ffffffff8116d7a0>] SyS_mkdirat+0x60/0xe0
   [<ffffffff8116d839>] SyS_mkdir+0x19/0x20
   [<ffffffff814c960d>] tracesys+0xcf/0xd4
  Code: ad 70 ff ff ff 48 89 9d 60 ff ff ff 4d 89 d5 4c 8b bd 68 ff ff ff 4c 8b 65 88 eb 50 0f 1f 00 48 8b 43 18 a8 03 0f 85 6c 03 00 00 <ff> 00 e8 1d 0a fb ff 85 c0 74 0d 80 3d f0 45 a1 00 00 0f 84 4c
  RIP  [<ffffffff810b37cc>] cgroup_mkdir+0x37c/0x740
   RSP <ffff88007f865df8>
  CR2: 0000000000000000
  ---[ end trace a4b14b49bc46fd60 ]---

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Acked-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>