LinuxKPI: remove < 5.0 version support

We are not aware of any out-of-tree consumers anymore
which would need KPI support for before Linux version 5.
Update the two in-tree consumers to use the new KPI.
This allows us to remove the extra version check and
will also give access to {lower,upper}_32_bits() unconditionally.

Sponsored-by: The FreeBSD Foundation
Reviewed-by: hselasky, rlibby, rstone
MFC-after: 2 weeks
X-MFC: to 13 only
Differential Revision: https://reviews.freebsd.org/D29391