tasks, sched/core: With a grace period after finish_task_switch(), remove unnecessary code

Remove work arounds that were written before there was a grace period
after tasks left the runqueue in finish_task_switch().

In particular now that there tasks exiting the runqueue exprience
a RCU grace period none of the work performed by task_rcu_dereference()
excpet the rcu_dereference() is necessary so replace task_rcu_dereference()
with rcu_dereference().

Remove the code in rcuwait_wait_event() that checks to ensure the current
task has not exited.  It is no longer necessary as it is guaranteed
that any running task will experience a RCU grace period after it
leaves the run queueue.

Remove the comment in rcuwait_wake_up() as it is no longer relevant.

Ref: 8f95c90ceb ("sched/wait, RCU: Introduce rcuwait machinery")
Ref: 150593bf86 ("sched/api: Introduce task_rcu_dereference() and try_get_task_struct()")
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/87lfurdpk9.fsf_-_@x220.int.ebiederm.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

include/linux/sched/task.h

@@ -119,7 +119,6 @@ static inline void put_task_struct(struct task_struct *t)
 		__put_task_struct(t);
 }
 
-struct task_struct *task_rcu_dereference(struct task_struct **ptask);
 void put_task_struct_rcu_user(struct task_struct *task);
 
 #ifdef CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT