ofproto-dpif: Correctly refresh all ports on ENOBUFS from dpif_port_poll().

ofproto-dpif: Correctly refresh all ports on ENOBUFS from dpif_port_poll().

dpif_port_poll() is allowed to return ENOBUFS if something might have
changed, but the specific change isn't easily reportable.  type_run()
didn't handle this case, so it wouldn't notice any changes when this
happened.

dpif-netdev (including dpif-dummy) uses ENOBUFS exclusively to report
changes, so this fixes a problem there.  dpif-linux rarely uses ENOBUFS but
it can do so if a kernel-to-user Netlink buffer overflows.

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