DP_SUCCESS() macro as inverse of DP_FAILED()

[PlagueCZ]

The absolute majority of calls are fail-early scenarios with DP_FAILED() as a wrapper.

However some calls are better handled via positive test, like

if (!DP_FAILED(rte_hash_lookup())) {
    // do extra code when entry already present
}

This is visually almost identical to the inverse case dues to how small the exclamation mark is.

It would be better to also have a DP_SUCCESS(ret) macro to make it absolutely clear that this is the non-standard test for positive result.