Fix callout_tickstofirst() behavior after signed integer ticks overflow. This should fix callout precision drop to 1/4s after 25 days of uptime with HZ = 1000.
Fix callout_tickstofirst() behavior after signed integer ticks overflow.
This should fix callout precision drop to 1/4s after 25 days of uptime
with HZ = 1000.

Submitted by:	Taku YAMAMOTO <taku@tackymt.homeip.net>

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