netlink: fix routing-policy-rule netlink type for FRA_GOTO/FRA_UNUSED2

netlink: fix routing-policy-rule netlink type for FRA_GOTO/FRA_UNUSED2

- RTA_OIF has no business in the routing-rule policy. It is numerical
  identical to FRA_GOTO. Fix using the correct enum value. Note that
  RTA_OIF/FRA_GOTO was not used by networkd, and the type was already
  correct at uint32. So, there is no change in behavior.

- RTA_GATEWAY also does not belong to the routing-rules. It is numerical
  identical to FRA_UNUSED2. Obviously, that value is unused as well,
  so there is no actual change in behavior either. In particular
  that is because:
    - kernel would not send messages with FRA_UNUSED2 attribute.
    - networkd would not try to parse/send RTA_GATEWAY/FRA_UNUSED2
      attributes.

Fixes: bce67bbee359eec19e6778619b6651100a1c1477