rcu: Remove unused stop-machine #include

Long ago, RCU used the stop-machine mechanism to implement expedited
grace periods, but no longer does so.  This commit therefore removes
the no-longer-needed #includes of linux/stop_machine.h.

Link: https://lwn.net/Articles/805317/


Reported-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>