odp-util: Convert flow serialization parameters to a struct.

odp-util: Convert flow serialization parameters to a struct.

Serializing between userspace flows and netlink attributes currently
requires several additional parameters besides the flows themselves.
This will continue to grow in the future as well. This converts
the function arguments to a parameters struct, which makes the code
easier to read and allowing irrelevant arguments to be omitted.

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