zio: remove io_cmd and DKIOCFLUSHWRITECACHE

There's no other options, so we can just always assume its a flush.

Includes some light refactoring where a switch statement was doing
control flow that no longer works.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16064