FreeBSD: Sync taskq_cancel_id() returns with Linux

Couple places in the code depend on 0 returned only if the task was
actually cancelled. Doing otherwise could lead to extra references
being dropped. The race could be small, but I believe CI hit it
from time to time.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by: iXsystems, Inc.
Closes #16565