mptcp: use "middlebox interference" RST when no DSS

RFC8684 suggests use of "Middlebox interference (code 0x06)" in case of
fully established subflow that carries data at TCP level with no DSS
sub-option.

This is generally the case when mpext is NULL or mpext->use_map is 0:
use a dedicated value of 'mapping_status' and use it before closing the
socket in subflow_check_data_avail().

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/518


Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20241021-net-next-mptcp-misc-6-13-v1-4-1ef02746504a@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>