xfrm: Convert struct xfrm_dst_lookup_params -> tos to dscp_t.

Add type annotation to the "tos" field of struct xfrm_dst_lookup_params,
to ensure that the ECN bits aren't mistakenly taken into account when
doing route lookups. Rename that field (tos -> dscp) to make that
change explicit.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>