xfrm: Convert xfrm_dst_lookup() to dscp_t.

Pass a dscp_t variable to xfrm_dst_lookup(), instead of an int, to
prevent accidental setting of ECN bits in ->flowi4_tos.

Only xfrm_bundle_create() actually calls xfrm_dst_lookup(). Since it
already has a dscp_t variable to pass as parameter, we only need to
remove the inet_dscp_to_dsfield() conversion.

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