JSON: Fix class values for mirrored special vdevs

This fixes things so mirrored special vdevs report themselves as
"class=special" rather than "class=normal".

This happens due to the way the vdev nvlists are constructed:

mirrored special devices - The 'mirror' vdev has allocation bias as
"special" and it's leaf vdevs are "normal"

single or RAID0 special devices - Leaf vdevs have allocation bias as
"special".

This commit adds in code to check if a leaf's parent is a "special"
vdev to see if it should also report "special".

Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Umer Saleem <usaleem@ixsystems.com>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #16217