loopback: Flush asyncmsgq from the right context

loopback: Flush asyncmsgq from the right context

u->asyncmsg is accessed from two IO threads. teardown() shouldn't
flush the queue from the main thread while both IO threads are still
potentially using the queue. This patch fixes that error by flushing
the queue from the sink input thread when the sink input is being
unlinked.

Flushing the queue in teardown() caused this assertion in
pa_asyncmsgq_get() to crash sometimes: pa_assert(!a->current)