Discard OpenFlow messages not for us in dpif_recv_openflow().
Currently there is only a single Netlink multicast group that is shared
by all OpenFlow datapaths. This is undesirable and should be fixed, but
a solution in every case is impossible due to Linux kernel limitations.
This commit works around the problem by dropping OpenFlow messages that
are related to a datapath that we are not interested in.
This should fix vswitchd behavior when more than one bridge is configured.
Thanks to Keith and Justin for diagnosing the problem.