Replace process_pending_timers() with process_pending_softirqs().

Replace process_pending_timers() with process_pending_softirqs().

This ensures that any critical softirqs are handled in a timely manner
(e.g., TIME_CALIBRATE_SOFTIRQ) while still avoiding being preempted by
the scheduler (by SCHEDULE_SOFTIRQ), which is the reason for avoiding
use of do_softirq() directly.

Signed-off-by: Keir Fraser <keir.fraser@citrix.com>