mbuf: remove the rte_pktmbuf structure

mbuf: remove the rte_pktmbuf structure

The rte_pktmbuf structure was initially included in the rte_mbuf
structure. This was needed when there was 2 types of mbuf (ctrl and
packet). As the control mbuf has been removed, we can merge the
rte_pktmbuf into the rte_mbuf structure.

Advantages of doing this:
  - the access to mbuf fields is easier (ex: m->data instead of m->pkt.data)
  - make the structure more consistent: for instance, there was no reason
    to have the ol_flags field in rte_mbuf
  - it will allow a deeper reorganization of the rte_mbuf structure in the
    next commits, allowing to gain several bytes in it

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
[Bruce: updated for latest code and new example apps]
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>