sched/deadline: check start_dl_timer expiry with ktime_before()

Replace ktime_us_delta() < 0 with ktime_before() so the past-expiry
test uses the same resolution as act/now.

Signed-off-by: Liang Hao <haohlliang@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://patch.msgid.link/20260816032643.44969-1-haohlliang@gmail.com