Fix generic cgroup subsystem permission checks

In 53b5e2f generic cgroup subsystem permission checks have been added.
When this is been done within procs_write an empty taskset is added to
the tasks css set. When a task later on migrates to a new group we see a
dmesg warning cause the mg_node isn't empty (cgroup.c:2086). Cause this
happens all the time this spams dmesg.

I am not really familiar with this code, but it looks to me like adding
the taskset is just a temporary action in this context. Therefore this
taskset should be removed after the actual check. This is what this fix
does.

This problem was seen and the fix tested on x86 using l-mr1 and master.

Change-Id: I9894d39e8b5692ef65149002b07e65a84a33ffea
Signed-off-by: Christian Poetzsch <christian.potzsch@imgtec.com>