kref: switch internal type to atomic_t and bring back const to kref_read

This unbreak drm-kmod build.

the const is part of Linux API

Unfortunately drm-kmod uses hand-rolled refcount* calls on a kref
object. For now go the easy route of keeping it operational by casting
stuff internally.

The general goal here is to make FreeBSD refcount API use an opaque
type, hence the ongoing removal of hand-rolled accesses.

Reported by: emaste