snat allocation function refactor

[PlagueCZ]

For #697 I need to re-use SNAT create/delete functions.

This PR reorganizes the code slightly for that to be easier in a followup PR.

I have separated the work into small commits so that each is easier to be reviewed as a separate change.

[PlagueCZ]

Oh, the "edge case" is the fact that when previously doing a rollback (which basically never happens) portmap_data->flow_cnt++; was not reverted.

[PlagueCZ]

I added a commit using a goto to a common return code because returning a positive result from the middle of the function is not common in our codebase and also any additional stuff needed to be done when returning the NAT port would need to be duplicate.

For example - changes from #700 would require duplicating DP_STATS_NAT_INC_USED_PORT_CNT(port).

Also for #697 there needs to be some dp_send_add_nat_to_other_dpservice() which would also then need to be done in two places.

That's why I opted for a simple goto to the return code. This can also be replaces by a wrapper function if preferred.

[PlagueCZ]

I had to rebase onto #700 because otherwise the review is really misleading as you never saw the actual code that I am working on.

[guvenc]

Thanks