ethdev: support optional Rx and Tx callbacks

ethdev: support optional Rx and Tx callbacks

Add optional support for inline processing of packets inside the RX
or TX call. For an RX callback, what happens is that we get a set of
packets from the NIC and then pass them to a callback function, if
configured, to allow additional processing to be done on them, e.g.
filling in more mbuf fields, before passing back to the application.
On TX, the packets are similarly post-processed before being handed
to the NIC for transmission.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>