odp-util: Return exact mask if netlink mask attribute is missing.

odp-util: Return exact mask if netlink mask attribute is missing.

In the ODP context an empty mask netlink attribute usually means that
the flow should be an exact match.

odp_flow_key_to_mask{,_udpif}() instead return a struct flow_wildcards
with matches only on recirc_id and vlan_tci.

A more appropriate behavior is to handle a missing (zero length) netlink
mask specially (like we do in userspace and Linux datapath) and create
an exact match flow_wildcards from the original flow.

This fixes a bug in revalidate_ukey(): every flow created with
megaflows disabled would be revalidated away, because the mask would
seem too generic. (Another possible fix would be to handle the special
case of a missing mask in revalidate_ukey(), but this seems a more
generic solution).