files_cgroup: fix error pointer when kvm_vm_worker_thread

hulk inclusion
category: bugfix
bugzilla: NA
CVE: NA

---------------------------

When fix CVE-2018-12207, the kvm_vm_worker_thread will attach all
cgroup subsystem. But the files cgroup doesn't support kernel thread.

Because the init_files doesn't init the files cgroup, when kernel thread
'kvm_vm_worker_thread' attach the files cgroup, the files_cgroup
get from 'init_files' is an error pointer. It lead the kernel panic
as below:
  [  724.842302]  page_counter_uncharge+0x1d/0x30
  [  724.842431]  files_cgroup_attach+0x7c/0x130
  [  724.842564]  ? css_set_move_task+0x12e/0x230
  [  724.842694]  cgroup_migrate_execute+0x2f9/0x3b0
  [  724.842833]  cgroup_attach_task+0x156/0x200
  [  724.843010]  ? kvm_mmu_pte_write+0x490/0x490 [kvm]
  [  724.843153]  cgroup_attach_task_all+0x81/0xd0
  [  724.843289]  ? __schedule+0x294/0x910
  [  724.843419]  kvm_vm_worker_thread+0x4a/0xc0 [kvm]
  [  724.843579]  ? kvm_exit+0x80/0x80 [kvm]
  [  724.843690]  kthread+0x112/0x130
  [  724.843792]  ?kthread_create_worker_on_cpu+0x70/0x70
  [  724.843948]  ret_from_fork+0x35/0x40

So, we add some check, if the task is kernel thread (files is
'init_files'), we doesn't do the more operation about the
files cgroup.

Fixes: baa10bc24e1e ("kvm: Add helper function for creating VM ...")
Signed-off-by: Zhang Xiaoxu <zhangxiaoxu5@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>