mbuf: replace data pointer by an offset

mbuf: replace data pointer by an offset

The mbuf structure already contains a pointer to the beginning of the
buffer (m->buf_addr). It is not needed to use 8 bytes again to store
another pointer to the beginning of the data.

Using a 16 bits unsigned integer is enough as we know that a mbuf is
never longer than 64KB. We gain 6 bytes in the structure thanks to
this modification.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
* Updated to apply to latest on mainline.
* Disabled vector PMD in config as it relies heavily on the mbuf layout
  This will be re-enabled in a subsequent commit once vPMD has been
  reworked to take account of mbuf changes.

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