mbuf: add a function to linearize a packet

mbuf: add a function to linearize a packet

This patch adds function rte_pktmbuf_linearize to let crypto PMD coalesce
chained mbuf before crypto operation and extend their capabilities to
support segmented mbufs when device cannot handle them natively.

Included unit tests for rte_pktmbuf_linearize functionality:

 1) Creates banch of segmented mbufs with different size and number of
    segments.
 2) Fills noncontigouos mbuf with sequential values.
 3) Uses rte_pktmbuf_linearize to coalesce segmented buffer into one
    contiguous.
 4) Verifies data in linearized buffer.

Signed-off-by: Tomasz Kulasek <tomaszx.kulasek@intel.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>