mptcp: set remote_deny_join_id0 on SYN recv

commit 96939cec upstream.

When a SYN containing the 'C' flag (deny join id0) was received, this
piece of information was not propagated to the path-manager.

Even if this flag is mainly set on the server side, a client can also
tell the server it cannot try to establish new subflows to the client's
initial IP address and port. The server's PM should then record such
info when received, and before sending events about the new connection.

Fixes: df377be3 ("mptcp: add deny_join_id0 in mptcp_options_received")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250912-net-mptcp-pm-uspace-deny_join_id0-v1-1-40171884ade8@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>
[ Conflicts in subflow.c, because of differences in the context, e.g.
  introduced by commit 3a236aef ("mptcp: refactor passive socket
  initialization"), which is not in this version. The same lines --
  using 'mptcp_sk(new_msk)' instead of 'owner' -- can still be added
  approximately at the same place, before calling
  mptcp_pm_new_connection(). ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>