[2.2.5-only] Make 'rmmod zfs' work after zfs-2.2.4 (#16406)

db65272ae was added to zfs-2.2.4 to stub in the
VDEV_PROP_RAIDZ_EXPANDING enum without adding the RAIDz expansion
feature. This was needed to provide the right enum count for when the
VDEV_PROP_SLOW_IO proprieties got added. This had the unfortunate side
effect of breaking module removal though.

Specifically, with the VDEV_PROP_RAIDZ_EXPANDING stub added,
the module would correctly omit making kobjects for the RAIDz expansion
vdev property, but then would try to blindly remove its non-existent
kobjects during module unload.

This commit fixes the issue by checking for an uninitialized kobject.

Fixes: #16249

Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>