When we crash, we'll stop the scheduler before we call the
shutdown_post_sync event. For adashutdown, this causes problems
because we need to poll for completion of the commands, but we're not
yet officially dumping yet, so the code from r326964 assumed we could
use the interrupt-driven commands rather than the polled ones. This
lead to a hang. Prevent this by also checking to see if the scheduler
is stopped to do the polling.

Reported by: markj@
Sponsored by: Netflix
Differential Review: https://reviews.freebsd.org/D13845