watchdog: softdog: fire watchdog even if softirqs do not get to run

Checking for timer expiration is done from the softirq TIMER_SOFTIRQ.

Since commit 4cd13c21

 ("softirq: Let ksoftirqd do its job"),
pending softirqs are no longer always handled immediately, instead,
if there are pending softirqs, and ksoftirqd is in state TASK_RUNNING,
the handling of the softirqs are deferred, and are instead supposed
to be handled by ksoftirqd, when ksoftirqd gets scheduled.

If a user space process with a real-time policy starts to misbehave
by never relinquishing the CPU while ksoftirqd is in state TASK_RUNNING,
what will happen is that all softirqs will get deferred, while ksoftirqd,
which is supposed to handle the deferred softirqs, will never get to run.

To make sure that the watchdog is able to fire even when we do not get
to run softirqs, replace the timers with hrtimers.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>