xfrm: Fix stack-out-of-bounds with misconfigured transport mode policies.

On policies with a transport mode template, we pass the addresses
from the flowi to xfrm_state_find(), assuming that the IP addresses
(and address family) don't change during transformation.

Unfortunately our policy template validation is not strict enough.
It is possible to configure policies with transport mode template
where the address family of the template does not match the selectors
address family. This lead to stack-out-of-bound reads because
we compare arddesses of the wrong family. Fix this by refusing
such a configuration, address family can not change on transport
mode.

We use the assumption that, on transport mode, the first templates
address family must match the address family of the policy selector.
Subsequent transport mode templates must mach the address family of
the previous template.

Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>