rcu: Reinstate synchronize_rcu_mult()

With the advent and likely usage of synchronize_rcu_rude(), there is
again a need to wait on multiple types of RCU grace periods, for
example, call_rcu_tasks() and call_rcu_tasks_rude().  This commit
therefore reinstates synchronize_rcu_mult() in order to allow these
grace periods to be straightforwardly waited on concurrently.

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