wg: Use atomic(9) instead of concurrency-kit atomics.

Kernel sanitizers only support atomic(9) operations.

Reviewed by: kevans, markj, emaste
Reported by: markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36910

(cherry picked from commit e32e1a160e01a5a0ca9e0b76ace37e58dc1379b5)