netdev-linux: Fix blocking while sending packets.

The AF_PACKET socket needs to be in nonblocking mode or trying to send
a packet can take a long time.

lib/netdev-linux.c

@@ -452,6 +452,7 @@ netdev_linux_init(void)
                 VLOG_ERR("failed to create packet socket: %s",
                          strerror(status));
             }
+            set_nonblocking(af_packet_sock);
         }
 
         /* Create rtnetlink socket. */