linuxkpi: refcount: Use atomic_t directly

Simply use a typedef for refcount_t on atomic_t, this allow us
to use a nativ type and also changing struct kref to directly use
a refcount_t like Linux.

Reviewed by: bz
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D45207