offloading: don't set multiple times per interface

This could happen with netmap igb0->igb0^ IPS mode.

src/util-ioctl.c

@@ -691,6 +691,9 @@ int GetIfaceOffloading(const char *dev, int csum, int other)
 
 int DisableIfaceOffloading(LiveDevice *dev, int csum, int other)
 {
+    /* already set */
+    if (dev->offload_orig != 0)
+        return 0;
 #if defined HAVE_LINUX_ETHTOOL_H && defined SIOCETHTOOL
     return DisableIfaceOffloadingLinux(dev, csum, other);
 #elif defined SIOCSIFCAP