sound: Drain buffer selinfo in sndbuf_free()

Prevent a use-after-free in kern_poll() by making sure the buffer's
selinfo is drained. This is required for a subsequent patch that
implements asynchronous audio device detach.

Reported by: KASAN
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D44544