mbuf: reorder fields by time of use

mbuf: reorder fields by time of use

*  Reorder the fields in the mbuf so that we have fields that are used
together side-by-side in the structure. This means that we have a
contiguous block of 8-bytes in the mbuf which are used to reset an mbuf
of descriptor rearm, and a block of 16-bytes of data (excluding flags)
which are set on RX from the received packet descriptor.
* Use dummy fields as appropriate to ensure alignment or to reserve gaps
for later field additions.
* Place most items which are not used by fast-path RX separately at the end
of the structure so they can later be moved to a separate cache line.
[The l2/l3 length fields are not moved at this stage as doing so will
cause overflow to the next cache line].

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>