In case we do not refill, we can overwrite cur pointer from prio_head
by one from not prioritized head, what looks as something that was
not intended.

This change make we always take works from prio_head first until it's
not empty.
Signed-off-by: NStanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: NJosef Bacik <jbacik@fb.com>