netdev-port: Fix invalid memory access in netdev_vport_poll_add().

netdev-port: Fix invalid memory access in netdev_vport_poll_add().

shash_find_data() returns an shash_node's 'data' member, but this code here
wants the shash_node itself, so it needs to use shash_find() instead.

This bug meant that any attempt to add a single netdev_vport to more than
one netdev_monitor would cause a segmentation fault.  Here's an example
command that reproduces it reliably for me under valgrind (because ofproto
always monitors its ports and the bridge monitors bond interfaces):

ovs-vsctl -- add-bond br0 bond0 p0 p1 \
          -- set interface p0 type=patch options:peer=p1 \
          -- set interface p1 type=patch options:peer=p0

Bug #4527.
Reported-by: Krishna Miriyala <krishna@nicira.com>