LinuxKPI: fix READ_ONCE() -Wcast-equal warnings

Rather than using ACCESS_ONCE() in READ_ONCE() add a missing cast
to const in order to satisfy -Wcast-equal by gcc.
Sadly we cannot do the same to WRITE_ONCE() which still is very
noisy.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D39706