rcu/tasks: Mark callbacks not currently participating in barrier operation

Each Tasks RCU flavor keeps a count of the number of callbacks that the
current rcu_barrier_tasks*() is waiting on, but there is currently no
easy way to work out which callback is stuck.  One way to do this is to
mark idle RCU-barrier callbacks by making the ->next pointer point to
the callback itself, and this commit does just that.

Later commits will use this for debug output.

Signed-off-by: "Paul E. McKenney" <paulmck@kernel.org>
Signed-off-by: Neeraj Upadhyay <neeraj.upadhyay@kernel.org>