net/rds: NULL pointer de-reference in rds_ib_add_one()

The parent field of a struct device may be NULL.  The macro
ibdev_to_node() should check for that.

Signed-off-by: Ka-Cheong Poon <ka-cheong.poon@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>