BUG/MINOR: activity: take care of late wakeups in "show tasks"

BUG/MINOR: activity: take care of late wakeups in "show tasks"

During the call to thread_isolate(), some other threads might have
performed some task_wakeup() which will have a call date past the
one we retrieved. It could be avoided by taking the current date
once we're alone but this would significantly affect the latency
measurements by adding the isolation time. Instead we're now only
accounting positive times, so that late wakeups normally appear
with a zero latency.

No backport is needed, this is 2.4.