dsp_oss_audioinfo() drops the topology lock and then dereferences the
softc, which device_detach() may have freed in the meantime.
Hold the topology lock for the whole function, like the other
SNDCTL_*INFO handlers do. Busying the device instead is not an option,
because dsp_ioctl() has already busied the one the ioctl was issued on,
which may be the same.
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation