mdesc: fix a missing-check bug in get_vdev_port_node_info()

In get_vdev_port_node_info(), 'node_info->vdev_port.name' is allcoated
by kstrdup_const(), and it returns NULL when fails. So
'node_info->vdev_port.name' should be checked.

Signed-off-by: Gen Zhang <blackgod016574@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>