secchan: Avoid dynamic allocation in xlate_actions().

secchan: Avoid dynamic allocation in xlate_actions().

The ofproto code currently does malloc()/realloc()/free() whenever it needs
to translate OpenFlow actions into datapath actions.  This is more or less
OK as-is, but the next commits will start keeping the datapath actions as
part of the rule.  That will require either wasting memory (because we
e.g. double the size of the malloc()'d buffer each realloc()) or making
a new malloc()'d copy of already malloc()'d memory.  Both solutions seem
wasteful.

So this commit instead prepares by keeping the ODP actions on-stack while
accumulating them, with a fixed maximum size that is the same maximum
used by the kernel datapath anyhow.