Fix bug in IP wildcarding.

    According to the OpenFlow specification, the mask on nw_src and nw_dst
    fields is not a description of how many bits to match, but rather a
    description of how many bits to mask. Hence, /0 is exact match and /32
    is a total wildcard.

    We had this backwards in the OpenFlow0x01 controller.

    This pull requests fixes the bug.