ofproto-dpif: Avoid searching all subfacets when creating first in a facet.

ofproto-dpif: Avoid searching all subfacets when creating first in a facet.

When we create the first subfacet within a facet, we know that there
cannot be an existing subfacet with the same key, so we can skip the search
through the ofproto's table of subfacets.

This is a small optimization, but it should not affect the flow setup rate
in most benchmarks, because in the stressful situations that benchmarks
create, OVS does not set up flows.

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