datapath: Fix VLAN-related kernel OOPS on XenServer.

datapath: Fix VLAN-related kernel OOPS on XenServer.

In deleting internal ports (other than the local port) we were failing to
call dp_del_if_hook even though we had called dp_add_if_hook when we added
it.  This prevented the sysfs kobject from being released and caused the
wrong address to be passed to kfree.  The former could cause random
memory corruption; the latter may be benign since the address was still in
the same slab object.