buf_ring: Consistently use atomic_*_32

We are operating on uint32_t values, use uint32_t atomic functions.

Reviewed by: alc, imp, kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46150

(cherry picked from commit 17a597bc13aa59ee90facaf9b8dada80f32eb52d)