Ensure dsl scan prefetch queue is emptied

This patch simply ensures that scn->scn_prefetch_queue is emptied
before the kernel module is unloaded and when scanning completes.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alek Pinchuk <apinchuk@datto.com>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8178