flush: only detect lack of flush support in one place

It seems there's no good reason for vdev_disk & vdev_geom to explicitly
detect no support for flush and set vdev_nowritecache. Instead, just
signal it by setting the error to ENOTSUP, and let zio_vdev_io_assess()
take care of it in one place.

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 #16855