filt_timerexpire: do not iterate over the interval

User-supplied data might make this loop too time-consuming. Divide
directly, and handle both the possibility that we were woken up earlier,
and arithmetic overflows/underflows from the calculation.

Reported and tested by: pho (previous version)
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D30069