ofproto-dpif: Handle failed flow 'put's.

ofproto-dpif: Handle failed flow 'put's.

If a flow cannot be installed in the datapath, we should notice
this and not treat it as installed.  This becomes an issue with
megaflows, since a batch of unique flows may come in that generate
a single new datapath megaflow that covers them.  Since userspace
doesn't know whether the datapath supports megaflows, each unique
flow will get a separate flow entry (which overlap when masks are
applied) and all except the first will get rejected by a megaflow-
supporting datapath as duplicates.

Signed-off-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>