memcgroup: remove a useless VM_BUG_ON()

Remove this VM_BUG_ON(), as Balbir stated:

We used to have a for loop with !list_empty() as a termination condition
and VM_BUG_ON(!pc) is a spill over.  With the new loop, VM_BUG_ON(!pc) does
not make sense.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Balbir Singh <balbir@in.ibm.com>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>