odp-util: Correctly [de]serialize mask for ND attributes.

odp-util: Correctly [de]serialize mask for ND attributes.

When converting between ODP attributes and struct flow_wildcards, we
check that all the prerequisites are exact matched on the mask.

For ND(ICMPv6) attributes, an exact match on tp_src and tp_dst
(which in this context are the icmp type and code) shold look like
htons(0xff), not htons(0xffff).  Fix this in two places.

The consequences were that the ODP mask wouldn't include the ND
attributes and the flow would be deleted by the revalidation.