rcutorture: Pull callback forward-progress data into rcu_fwd struct

Now that RCU behaves reasonably well with the current single-kthread
call_rcu() forward-progress testing, it is time to add more kthreads.
This commit takes a first step towards that goal by wrapping what
will be the per-kthread data into a new rcu_fwd structure.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>