net/iavf: fix VLAN extraction in AVX512 path

net/iavf: fix VLAN extraction in AVX512 path

The new VIRTCHNL_VF_OFFLOAD_VLAN_V2 capability added support that allows
the PF to set the location of the RX VLAN tag for stripping offloads.

So the VF needs to extract the VLAN tag according to the location flags.

This patch is the fix for AVX512 path, as AVX2 is already fixed.

Fixes: 9c9aa0040344 ("net/iavf: add offload path for Rx AVX512 flex descriptor")
Signed-off-by: Leyi Rong <leyi.rong@intel.com>
Tested-by: Qin Sun <qinx.sun@intel.com>