kernel/work: Fix race under with delayed work item cancellation

The call to unschedule_locked() would return true ("successfully
unscheduled") even in the case where the underlying z_abort_timeout()
failed (because the callback was already unpended and
in-progress/complete/about-to-be-run, remember that timeout callbacks
are unsynchronized), leading to state bugs and races against the
callback behavior.

Correctly detect that case and propagate the error to the caller.

Fixes #51872

Signed-off-by: Andy Ross <andyross@google.com>