zvol: remove void return casts on void-returning functions

Casting unused returns to (void) is already of dubious value, but it's
entirely meaningless on functions that are defined as void return.
Remove the clutter.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Reviewed-by: Fedor Uporov <fuporov.vstack@gmail.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Railway Corporation
Closes #17596