dp-packet: Init specific mbuf fields.

dp-packet: Init specific mbuf fields.

dp_packets are created using xmalloc(); in the case of OvS-DPDK, it's
possible the the resultant mbuf portion of the dp_packet contains
random data. For some mbuf fields, specifically those related to
multi-segment mbufs and/or offload features, random values may cause
unexpected behaviour, should the dp_packet's contents be later copied
to a DPDK mbuf. It is critical therefore, that these fields should be
initialized to 0.

This patch ensures that the following mbuf fields are initialized to
appropriate values on creation of a new dp_packet:
   - ol_flags=0
   - nb_segs=1
   - tx_offload=0
   - packet_type=0
   - next=NULL

Adapted from an idea by Michael Qiu <qiudayu@chinac.com>:
https://patchwork.ozlabs.org/patch/777570/

Co-authored-by: Tiago Lam <tiago.lam@intel.com>
Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Signed-off-by: Tiago Lam <tiago.lam@intel.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>