mbuf: meta-data offset

mbuf: meta-data offset

Added zero-size field (offset in data structure) to specify the beginning
of packet meta-data in the packet buffer just after the mbuf.

The size of the packet meta-data is application specific and the packet
meta-data is managed by the application.

The packet meta-data should always be accessed through the provided macros.

This is used by the Packet Framework libraries (port, table, pipeline).

There is absolutely no performance impact due to this mbuf field, as it
does not take any space in the mbuf structure (zero-size field).

Signed-off-by: Cristian Dumitrescu <cristian.dumitrescu@intel.com>
Acked-by: Pablo de Lara Guarch <pablo.de.lara.guarch@intel.com>
Acked by: Ivan Boule <ivan.boule@6wind.com>