time: Handle kern_clock_gettime() failures in kern_clock_nanosleep()

kern_clock_gettime(CLOCK_TAI) can fail with EINVAL.

Reported by: syzbot+e17e46b1f0b65027b005@syzkaller.appspotmail.com
Reviewed by: imp, kib
Fixes: 7b7ba7857ce8 ("Implement CLOCK_TAI")
Differential Revision: https://reviews.freebsd.org/D51744