Streaming and the commit block job only want to apply throttling when
they actually copied data instead of skipping it, so they made the
calculation of delay_ns conditional. However, delay_ns isn't reset when
skipping some sectors, so instead of not waiting, the old delay is
applied again.

Properly reset delay_ns where needed.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>