kevent(2): Bugfix for wrong EVFILT_TIMER timeouts

When using NOTE_NSECONDS in the kevent(2) API, US_TO_SBT should be
used instead of NS_TO_SBT, otherwise the timeout results are
misleading.

PR: 252539
Reviewed by: kevans, kib
Approved by: kevans
Differential Revision: https://reviews.freebsd.org/D28067

(cherry picked from commit 4d0c33be634a929f323117f04e6b1670776f9e37)
(cherry picked from commit 6a3ad2d0a7b633bad2bb33f9c4c426dffcc91633)