tnl-ports: Generate mask with correct prerequisites.

tnl-ports: Generate mask with correct prerequisites.

We should match on the transport ports only if the tunnel has a UDP
header.  It doesn't make sense to match on transport port for GRE
tunnels.

Also, to match on fragment bits we should use FLOW_NW_FRAG_MASK instead
of 0xFF.  FLOW_NW_FRAG_MASK is what we get if we convert to the ODP
netlink format and back.

Adding the correct masks in the tunnel router classifier helps in making
sure that the translation generates masks that respect prerequisites.

If the mask has some fields that do not respect prerequisites, the flow
will get deleted by revalidation, because translating to ODP format and
back will generate a more generic mask, which will be perceived as too
generic (compared with the one generated by the translation).