netdev-dpdk: fix mbuf sizing

netdev-dpdk: fix mbuf sizing

There are numerous factors that must be considered when calculating
the size of an mbuf:
- the data portion of the mbuf must be sized in accordance With Rx
  buffer alignment (typically 1024B). So, for example, in order to
  successfully receive and capture a 1500B packet, mbufs with a
  data portion of size 2048B must be used.
- in OvS, the elements that comprise an mbuf are:
  * the dp packet, which includes a struct rte mbuf (704B)
  * RTE_PKTMBUF_HEADROOM (128B)
  * packet data (aligned to 1k, as previously described)
  * RTE_PKTMBUF_TAILROOM (typically 0)

Some PMDs require that the total mbuf size (i.e. the total sum of all
of the above-listed components' lengths) is cache-aligned. To satisfy
this requirement, it may be necessary to round up the total mbuf size
with respect to cacheline size. In doing so, it's possible that the
dp_packet's data portion is inadvertently increased in size, such that
it no longer adheres to Rx buffer alignment. Consequently, the
following property of the mbuf no longer holds true:

    mbuf.data_len == mbuf.buf_len - mbuf.data_off

This creates a problem in the case of multi-segment mbufs, where that
assumption is assumed to be true for all but the final segment in an
mbuf chain. Resolve this issue by adjusting the size of the mbuf's
private data portion, as opposed to the packet data portion when
aligning mbuf size to cachelines.

Co-authored-by: Tiago Lam <tiago.lam@intel.com>
Fixes: 4be4d22 ("netdev-dpdk: clean up mbuf initialization")
Fixes: 31b88c9 ("netdev-dpdk: round up mbuf_size to cache_line_size")
CC: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Signed-off-by: Mark Kavanagh <mark.b.kavanagh@intel.com>
Signed-off-by: Tiago Lam <tiago.lam@intel.com>
Acked-by: Santosh Shukla <santosh.shukla@caviumnetworks.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Ian Stokes <ian.stokes@intel.com>