net/iavf: support flex desc metadata extraction

net/iavf: support flex desc metadata extraction

Enable metadata extraction for flexible descriptors in AVF, that would
allow network function directly get metadata without additional parsing
which would reduce the CPU cost for VFs. The enabling metadata
extractions involve the metadata of VLAN/IPv4/IPv6/IPv6-FLOW/TCP/MPLS
flexible descriptors, and the VF could negotiate the capability of
the flexible descriptor with PF and correspondingly configure the
specific offload at receiving queues.

Signed-off-by: Jeff Guo <jia.guo@intel.com>
Acked-by: Haiyue Wang <haiyue.wang@intel.com>