reboot: move reboot_notifier_list to kernel/reboot.c

All the functions related to the reboot notifier list are in
kernel/reboot.c.  Move the list itself, too.  As there are no direct users
anymore, make the declaration static.

Link: https://lkml.kernel.org/r/20241012-reboot_notifier_list-v1-1-6093bb9455ce@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>