linux/rw_destroy: assert no holders before destroying

While rw_destroy() may do nothing on Linux, we still want to make sure
that we don't have any holders outstanding like we do for mutexes.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17718