exit: Sleep at TASK_IDLE when waiting for application core dump

Currently, the coredump_task_exit() function sets the task state
to TASK_UNINTERRUPTIBLE|TASK_FREEZABLE, which usually works well.
But a combination of large memory and slow (and/or highly contended)
mass storage can cause application core dumps to take more than
two minutes, which can cause check_hung_task(), which is invoked by
check_hung_uninterruptible_tasks(), to produce task-blocked splats.
There does not seem to be any reasonable benefit to getting these splats.

Furthermore, as Oleg Nesterov points out, TASK_UNINTERRUPTIBLE could
be misleading because the task sleeping in coredump_task_exit() really
is killable, albeit indirectly.  See the check of signal->core_state
in prepare_signal() and the check of fatal_signal_pending()
in dump_interrupted(), which bypass the normal unkillability of
TASK_UNINTERRUPTIBLE, resulting in coredump_finish() invoking
wake_up_process() on any threads sleeping in coredump_task_exit().

Therefore, change that TASK_UNINTERRUPTIBLE to TASK_IDLE.

Reported-by: Anhad Jai Singh <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Acked-by: Oleg Nesterov <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Christian Brauner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Chris Mason <[email protected]>
Cc: Rik van Riel <[email protected]>

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