cxgbe(4): Use two hashes instead of a table to keep track of
hashfilters. Two because the driver needs to look up a hashfilter by
its 4-tuple or tid.
A couple of fixes while here:
- Reject attempts to add duplicate hashfilters.
- Do not assume that any part of the 4-tuple that isn't specified is 0. This makes it consistent with all other mandatory parameters that already require explicit user input.