select: store end_time as timespec64 in restart block

select: store end_time as timespec64 in restart block

Storing the end time seconds as 'unsigned long' can lead to truncation
on 32-bit architectures if assigned from the 64-bit timespec64::tv_sec.
As the select() core uses timespec64 consistently, also use that in the
restart block.

This also allows the simplification of the accessors.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Link: https://patch.msgid.link/20251223-restart-block-expiration-v2-1-8e33e5df7359@linutronix.de
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>