vti: use right inner_mode for inbound inter address family policy checks

In case of inter address family tunneling (IPv6 over vti4 or IPv4 over
vti6), the inbound policy checks in vti_rcv_cb() and vti6_rcv_cb() are
using the wrong address family. As a result, all inbound inter address
family traffic is dropped.

Use the xfrm_ip2inner_mode() helper, as done in xfrm_input() (i.e., also
increment LINUX_MIB_XFRMINSTATEMODEERROR in case of error), to select the
inner_mode that contains the right address family for the inbound policy
checks.

Signed-off-by: Thomas Zeitlhofer <thomas.zeitlhofer+lkml@ze-it.at>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>