ofproto-dpif: Avoid extra flow copy in xlate_actions() for unneeded warnings.

ofproto-dpif: Avoid extra flow copy in xlate_actions() for unneeded warnings.

The copy of the extra flow copy here was showing up in profiles.  We only
need this copy if we end up doing a "trace" to warn the user.  Most runs
won't ever do that, so don't start making copies until we actually hit
such a case.

This has a small behavioral change in that we'll only get a warning on the
*second* time we hit the resubmit recursion limit, not on the first.  I
doubt that's really a problem.

Signed-off-by: Ben Pfaff <blp@nicira.com>