sysctl_dev_pcm_bitperfect(), sysctl_dev_pcm_eq() and
sysctl_dev_pcm_eq_preamp() check PCM_REGISTERED() and then busy the
device with nothing held in between, so pcm_unregister() can run and
destroy the sx before PCM_ACQUIRE() takes it. These sysctls are
registered in the device's newbus context, so they outlive
pcm_unregister().
Hold the topology lock across the check and the busy, as the other
handlers do.
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation