ofproto-dpif: Don't put new subfacets as result of "userspace" action.

Don't install a flow if it's the result of the "userspace" action for an
already installed facet.  This can occur when a datapath flow with
wildcards has a "userspace" action and flows sent to userspace result in
a different subfacet, which will then be rejected as overlapping by the
datapath.

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Co-authored-by: Ethan Jackson <ethan@nicira.com>