rcu: Rename rcu_check_callbacks() to rcu_sched_clock_irq()

The name rcu_check_callbacks() arguably made sense back in the early
2000s when RCU was quite a bit simpler than it is today, but it has
become quite misleading, especially with the advent of dyntick-idle
and NO_HZ_FULL.  The rcu_check_callbacks() function is RCU's hook into
the scheduling-clock interrupt, and is now but one of many ways that
callbacks get promoted to invocable state.

This commit therefore changes the name to rcu_sched_clock_irq(),
which is the same number of characters and clearly indicates this
function's relation to the rest of the Linux kernel.  In addition, for
the sake of consistency, rcu_flavor_check_callbacks() is also renamed
to rcu_flavor_sched_clock_irq().

While in the area, the header comments for both functions are reworked.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

include/linux/rcupdate.h

@@ -89,7 +89,7 @@ static inline int rcu_preempt_depth(void)
 /* Internal to kernel */
 void rcu_init(void);
 extern int rcu_scheduler_active __read_mostly;
-void rcu_check_callbacks(int user);
+void rcu_sched_clock_irq(int user);
 void rcu_report_dead(unsigned int cpu);
 void rcutree_migrate_callbacks(int cpu);