watchdog: core: make sure the watchdog_worker is not deferred

commit 4cd13c21 ("softirq: Let ksoftirqd do its job") has the
effect of deferring timer handling in case of high CPU load, hence
delaying the delayed work allthought the worker is running which
high realtime priority.

As hrtimers are not managed by softirqs, this patch replaces the
delayed work by a plain work and uses an hrtimer to schedule that work.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Guenter Roeck <Linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>