buf_ring: Support DEBUG_BUFRING in userspace

The only part of DEBUG_BUFRING we don't support in userspace is the
mutex checks. Add _KERNEL checks around these so we can enable the
extra debugging.

Reviewed by: alc, imp, kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D46149

(cherry picked from commit d3d34d56bee4222b3bf3ec26d7877998405115a3)