Fix: fencer: prevent watchdog fencing from timing out on insufficient per-device fencing timeout

A specified per-device fencing timeout shorter than
stonith-watchdog-timeout could be arguably considered a
misconfiguration. But given the configured stonith-watchdog-timeout,
it's pretty well known how long a watchdog fencing takes. In that case,
we should ideally tolerate the misconfiguration and allow watchdog
fencing to take as much time as it needs.