dpif-netdev: Properly track whether there is a vlan header.

dpif-netdev: Properly track whether there is a vlan header.

It looks to me like the current dpif-netdev implementation doesn't handle
the case where a packet comes in without a VLAN and then is subjected to
multiple ODPAT_SET_VLAN_* operations.  dp_netdev_modify_vlan_tci() just
checks the flow key each time to see whether there's a VLAN, but it doesn't
update the flow key to note that there is now a VLAN.

One fix would be to update the flow key, but it's "const" these days.
Instead, add a check for whether the Ethernet type is ETH_TYPE_VLAN,
which should be equivalent.