sched: fix iteration across timer list

We iterate over the timer list using an iterator, but the timer list can
change during iteration due to timers being re-inserted.

Switch to just looking at the head of the list instead, maintaining no
state across loop iterations.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Tested-by: Pekka Enberg <penberg@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>