rcu-tasks: Maintain real-time response in rcu_tasks_postscan()

The current code will scan the entirety of each per-CPU list of exiting
tasks in ->rtp_exit_list with interrupts disabled.  This is normally just
fine, because each CPU typically won't have very many tasks in this state.
However, if a large number of tasks block late in do_exit(), these lists
could be arbitrarily long.  Low probability, perhaps, but it really
could happen.

This commit therefore occasionally re-enables interrupts while traversing
these lists, inserting a dummy element to hold the current place in the
list.  In kernels built with CONFIG_PREEMPT_RT=y, this re-enabling happens
after each list element is processed, otherwise every one-to-two jiffies.

[ paulmck: Apply Frederic Weisbecker feedback. ]

Link: https://lore.kernel.org/all/ZdeI_-RfdLR8jlsm@localhost.localdomain/

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>