rcu: Don't invoke try_invoke_on_locked_down_task() with irqs disabled

The try_invoke_on_locked_down_task() function requires that
interrupts be enabled, but it is called with interrupts disabled from
rcu_print_task_stall(), resulting in an "IRQs not enabled as expected"
diagnostic.  This commit therefore updates rcu_print_task_stall()
to accumulate a list of the first few tasks while holding the current
leaf rcu_node structure's ->lock, then releases that lock and only then
uses try_invoke_on_locked_down_task() to attempt to obtain per-task
detailed information.  Of course, as soon as ->lock is released, the
task might exit, so the get_task_struct() function is used to prevent
the task structure from going away in the meantime.

Link: https://lore.kernel.org/lkml/000000000000903d5805ab908fc4@google.com/
Fixes: 5bef8da6

 ("rcu: Add per-task state to RCU CPU stall warnings")
Reported-by:  <syzbot+cb3b69ae80afd6535b0e@syzkaller.appspotmail.com>
Reported-by:  <syzbot+f04854e1c5c9e913cc27@syzkaller.appspotmail.com>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>