odp-util: Correctly generate wildcards when formating nested attributes.

odp-util: Correctly generate wildcards when formating nested attributes.

When formatting netlink attributes if no mask is present a wildcarded
attribute is synthesized for the purposes of later processing. In
the case of nested attributes this must be done recursively, filling
in the correct attributes at each level rather than just generating
a set of zeros of the correct size. This is done already but it
always uses the attribute type for the top level keys - this corresponds
to nested ENCAP attributes. However, we have several levels of potentially
nested attributes for tunnels that each have their own types.

This uses an approach similar to the kernel where we have sets of
tables for the type of each attribute linked together by pointers.
This allows the mask generation function to automatically traverse
the nested attributes and always get the right types.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>