rcu: Remove "cpu" argument to rcu_note_context_switch()

The "cpu" argument to rcu_note_context_switch() is always the current
CPU, so drop it.  This in turn allows the "cpu" argument to
rcu_preempt_note_context_switch() to be removed, which allows the sole
use of "cpu" in both functions to be replaced with a this_cpu_ptr().
Again, the anticipated cross-CPU uses of these functions has been
replaced by NO_HZ_FULL.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 0699add..501baa9 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -656,7 +656,7 @@
 		 * in the task stack here.
 		 */
 		__do_softirq();
-		rcu_note_context_switch(cpu);
+		rcu_note_context_switch();
 		local_irq_enable();
 		cond_resched();
 		return;