netdev-linux: Don't log vport warnings when kernel datapath not loaded.

The *_get_stats functions call get_stats_via_vport(), which tries to get
information about ports attached to the kernel datapath.  When a pure
userspace switch is used (eg, the OVS kernel module isn't loaded), ports
are not attached to a kernel datapath, so warnings get logged.  This
commit handles that case and doesn't log a warning.  However, if the
kernel datapath is loaded, ports attached to a userspace datapath will
still log a warning.

Signed-off-by: Justin Pettit <jpettit@nicira.com>

lib/netdev-linux.c

@@ -1347,7 +1347,7 @@ get_stats_via_vport(const struct netdev *netdev_,
         int error;
 
         error = netdev_vport_get_stats(netdev_, stats);
-        if (error) {
+        if (error && error != ENOENT) {
             VLOG_WARN_RL(&rl, "%s: obtaining netdev stats via vport failed "
                          "(%s)", netdev_get_name(netdev_), strerror(error));
         }