rcu-tasks: Don't remove tasks with pending IPIs from holdout list

Currently, the check_all_holdout_tasks_trace() function removes all tasks
marked with ->trc_reader_checked from the holdout list, including those
with IPIs pending.  This means that the IPI handler might arrive at
a task that has already been removed from the list, which is at best
an accident waiting to happen.

This commit therefore avoids removing tasks with IPIs pending from
the holdout list.  This in turn means that the "if" condition in the
for_each_online_cpu() loop in rcu_tasks_trace_postgp() should always
evaluate to false, so a WARN_ON_ONCE() is added to check that.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>