openvswitch: Add missing case OVS_TUNNEL_KEY_ATTR_PAD

Added a case for OVS_TUNNEL_KEY_ATTR_PAD to the switch statement
in ip_tun_from_nlattr in order to prevent the default case
returning an error.

Fixes: b46f6ded ("libnl: nla_put_be64(): align on a 64-bit area")
Signed-off-by: Kris Murphy <kriskend@linux.vnet.ibm.com>
Acked-by: Joe Stringer <joe@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>