sched/headers: Rename task_struct::state to task_struct::__state in the comments too

The rename in 2f064a59 ("sched: Change task_struct::state") missed the
comments.

[ mingo: Improved the changelog. ]

Signed-off-by: Chin Yik Ming <yikming2222@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Link: https://lore.kernel.org/r/20230717064952.2804-1-yikming2222@gmail.com