rcu: Update synchronize_rcu_mult() comment for call_rcu_hurry()
Those who have worked with RCU for some time will naturally think in
terms of the long-standing call_rcu() API rather than the much newer
call_rcu_hurry() API. But it is call_rcu_hurry() that you should normally
pass to synchronize_rcu_mult(). This commit therefore updates the header
comment to point this out.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org> Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>