net/tap: Set return code on failure

Match the other error handling in this function.

Fixes: e7b347d0bf6 ("net: detect errors from probing vnet hdr flag for TAP devices")

Reviewed-by: Patrick Venture <venture@google.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Peter Foley <pefoley@google.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>

net/tap.c

@@ -900,6 +900,7 @@ int net_init_tap(const Netdev *netdev, const char *name,
             if (i == 0) {
                 vnet_hdr = tap_probe_vnet_hdr(fd, errp);
                 if (vnet_hdr < 0) {
+                    ret = -1;
                     goto free_fail;
                 }
             } else if (vnet_hdr != tap_probe_vnet_hdr(fd, NULL)) {