LinuxKPI: switch jiffies and timer->expire to unsigned long

It seems these functions work with unsigned long and not int in Linux.
Start simply replacing the int where I came across it while debugging
a wireless driver timer modification. Also sprinkle in some "const".

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D48318