[zfs-2.2.8] Revert "vdev_disk: default to classic submission for 2.2.x"

This reverts commit eebf00bee91de2c7b7d03a9c1e5e2f3e5fd66c9e.

Classic mode is causing assertions on both this branch and master:

VERIFY3(page_count(page) > 0) failed (0 > 0)
PANIC at abd_os.c:1069:bio_map()

Signed-off-by: Tony Hutter <hutter2@llnl.gov>