net: store port/representator id in metadata_dst

Switches and modern SR-IOV enabled NICs may multiplex traffic from Port
representators and control messages over single set of hardware queues.
Control messages and muxed traffic may need ordered delivery.

Those requirements make it hard to comfortably use TC infrastructure today
unless we have a way of attaching metadata to skbs at the upper device.
Because single set of queues is used for many netdevs stopping TC/sched
queues of all of them reliably is impossible and lower device has to
retreat to returning NETDEV_TX_BUSY and usually has to take extra locks on
the fastpath.

This patch attempts to enable port/representative devs to attach metadata
to skbs which carry port id.  This way representatives can be queueless and
all queuing can be performed at the lower netdev in the usual way.

Traffic arriving on the port/representative interfaces will be have
metadata attached and will subsequently be queued to the lower device for
transmission.  The lower d...