MFC r264550: Fix VIRTUAL and PROF interval timers for short intervals, broken at r247903.
MFC r264550:
Fix VIRTUAL and PROF interval timers for short intervals, broken at r247903.

Due to the way those timers are implemented, we can't handle very short
intervals.  In addition to that mentioned patch caused math overflows
for short intervals.  To avoid that round those intervals to 1 tick.

PR:		kern/187668

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