[RFC ipsec-next 0/4]: Support multiple VTIs with the same src+dst pair

[Lorenzo Colitti <lorenzo@google.com>]

When using IPsec tunnel mode, VTIs provide many benefits compared
to direct configuration of xfrm policies / states. However, one
limitation is that there can only be one VTI between a given pair
of IP addresses. This does not allow configuring multiple IPsec
tunnels to the same security gateway. This is required by some
deployments, for example I-WLAN [3GPP TS 24.327].

This patchset introduces a new VTI_KEYED flag that allows
configuration of multiple VTIs between the same IP address
pairs. The output path is the same as current VTI behaviour,
where a routing lookup selects a VTI interface, and the VTI's
okey specifies the mark to use in the XFRM lookup. The input and
ICMP error paths instead work by first looking up an SA with a
loose match that ignores the mark. That mark is then used to find
the tunnel by ikey.

This approach is simple and requires few userspace changes, but
it has one limitation in that ICMP errors received in response to
VTI-emitted packets can only be processed if the VTI's ikey and
okey are the same. This limitation could be lifted by introducing
another XFRM mark, similar to XFRMA_OUTPUT_MARK, but used for
input.