sched_ext: Fix incorrect time delta calculation in time_delta()

When (s64)(after - before) > 0, the code returns the result of
(s64)(after - before) > 0 while the intended result should be
(s64)(after - before). That happens because the middle operand of
the ternary operator was omitted incorrectly, returning the result of
(s64)(after - before) > 0. Thus, add the middle operand
-- (s64)(after - before) -- to return the correct time calculation.

Fixes: d07be814 ("sched_ext: Add time helpers for BPF schedulers")
Signed-off-by: Changwoo Min <changwoo@igalia.com>
Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>