flow: save "vlan_hdrs" memset for untagged traffic

flow: save "vlan_hdrs" memset for untagged traffic

For untagged traffic, it is unnecessary to clear vlan_hdrs as it costs 32B
memset. So the patch improves it by postponing to clear vlan_hdrs until
ethtype check. It can benefit both untagged and single-tagged traffic. From
testing, it does not impact performance of dual-tagged traffic.

Reviewed-by: Gavin Hu <Gavin.Hu@arm.com>
Signed-off-by: Yanqin Wei <Yanqin.Wei@arm.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>