kevent: Fix an off-by-one in filt_timerexpire_l()

Suppose a periodic kevent timer fires close to its deadline, so that
now - kc->next is small. Then delta ends up being 1, and the next timer
deadline is set to (delta + 1) * kc->to, where kc->to is the timer
period. This means that the timer fires at half of the requested rate,
and the value returned in kn_data is similarly inaccurate.

PR: 264131
Fixes: 7cb40543e964 ("filt_timerexpire: do not iterate over the interval")
Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35313