buf_ring: Support DEBUG_BUFRING in userspace

[andrew]

Summary

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.

Sponsored by: Arm Ltd

[kib]

If this is useful in kernel, why hen why not use mutex from either normal libpthread or mtx from libstdthread? (The later seems to not have mtx_owned analog)

[andrew]

The mtx code appears to be a check that the correct mutex is being held by the kernel so only one thread can access the single-consumer functions at a time. In userspace the tests I wrote don't use a mutex as only one consumer thread is created when testing these functions.