linuxkpi: Add refcount_dec_and_test

In Linux this takes a refcount_t argument but in linuxkpi struct kref
uses an atomic_t for the refcount and code in drm directly uses this
function with a kref so use an atomic_t here.

Reviewed by: bz
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36099