xen-netback: use correct index for invalidation in xen_netbk_tx_check_gop()

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: stable@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>