Schedule wakeups asynchronously in rnd_extract_data.

Otherwise, rndsink_request takes rndsinks_lock and calls
rnd_extract_data, which synchronously calls rndsinks_distribute,
which takes rndsinks_lock -> boom.

This is a stop-gap on a stop-gap on a stop-gap; we really ought to
back out all of these stop-gaps, make bcm2835_rng call rnd_add_data
asynchronously to work around the original symptom, and design a real
solution when we have time to sort this mess out properly.

(riastradh)