mbuf: set mbuf fields while in pool

mbuf: set mbuf fields while in pool

Set the value of m->refcnt to 1, m->nb_segs to 1 and m->next
to NULL when the mbuf is stored inside the mempool (unused).
This is done in rte_pktmbuf_prefree_seg(), before freeing or
recycling a mbuf.

Before this patch, the value of m->refcnt was expected to be 0
while in pool.

The objectives are:

- to avoid drivers to set m->next to NULL in the early Rx path, since
  this field is in the second 64B of the mbuf and its access could
  trigger a cache miss

- rationalize the behavior of raw_alloc/raw_free: one is now the
  symmetric of the other, and refcnt is never changed in these functions.

To optimize the freeing of the segments, we try try to only update
m->refcnt, m->next, and m->nb_segs when it's required (idea from
Konstantin Ananyev <konstantin.ananyev@intel.com>).

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>