BUG: cgroup_task_counter subsys may crash with whole-threadgroupmove

[Ben Blum]

I was testing some patches for cgroup_attach_proc and managed to cause a
crash with the following usage pattern:

    mount -t cgroup none -o tasks /dev/cgroup
    cd /dev/cgroup
    mkdir foo
    echo $PID > foo/cgroup.procs
    echo $PID > tasks
    echo $PID > foo/cgroup.procs

Where $PID is the thread ID of a member of a multithreaded process (my
test program just does CLONE_THREAD 8 times and then all threads sleep).
(It doesn't matter if the thread is the group leader or not, but a
single-threaded process doesn't crash.)

And get the following kernel panic:
http://maximegalon.andrew.cmu.edu/cgroup-taskstats/panic.txt

It's deterministic, and happens only when the "tasks" subsystem is
mounted.

I'm using user-mode linux to test, with the following config:
http://maximegalon.andrew.cmu.edu/cgroup-taskstats/config.txt

and I ran it in GDB to get the following backtrace:
http://maximegalon.andrew.cmu.edu/cgroup-taskstats/bt.txt

Thanks,
Ben
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