net: fix TAP networking on host kernels without IFF_VNET_HDR support

vnet_hdr is initialized at 1 by default. We need to reset it to 0 if
the kernel doesn't support IFF_VNET_HDR.
Signed-off-by: NPierre Riteau <Pierre.Riteau@irisa.fr>
Signed-off-by: NMark McLoughlin <markmc@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

net/tap-linux.c

@@ -52,6 +52,8 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr, int vnet_hdr_required
             features & IFF_VNET_HDR) {
             *vnet_hdr = 1;
             ifr.ifr_flags |= IFF_VNET_HDR;
+        } else {
+            *vnet_hdr = 0;
         }
 
         if (vnet_hdr_required && !*vnet_hdr) {