vdev_disk: default to classic submission for 2.2.x

We don't want to change to brand-new code in the middle of a stable
series, but we want it available to test for people running into page
splitting issues.

This commits make zfs_vdev_disk_classic=1 the default, and updates the
documentation to better explain what's going on.

Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.