tcp: Set route scope properly in cookie_v4_check().

RT_CONN_FLAGS(sk) overloads flowi4_tos with the RTO_ONLINK bit when
sk has the SOCK_LOCALROUTE flag set. This allows
ip_route_output_key_hash() to eventually adjust flowi4_scope.

Instead of relying on special handling of the RTO_ONLINK bit, we can
just set the route scope correctly. This will eventually allow to avoid
special interpretation of tos variables and to convert ->flowi4_tos to
dscp_t.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>