sched: drop related_thread_group_lock before calling call_rcu()

When the last task is removed from a related thread group, the
group is freed by invoking an asynchronous RCU callback. The
call_rcu() can wakeup rcu, kswapd tasks. Hence drop the
related thread group lock, otherwise the wakeup path tries
to acquire the same lock and results in deadlock.

Change-Id: Idfb38a0276edbf6816369b3bab7f21bef5ccc556
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>