zio: rename ZIO_TYPE_IOCTL to ZIO_TYPE_FLUSH

The only possible ioctl is a flush, and any other kind of meta-operation
introduced in the future is likely to have different semantics (much
like trim did). So, lets just call it what it is.

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