Match the other error handling in this function.

Fixes: e7b347d0 ("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>