mptcp: Disallow MPTCP subflows from sockmap

The sockmap feature allows bpf syscall from userspace, or based on bpf
sockops, replacing the sk_prot of sockets during protocol stack processing
with sockmap's custom read/write interfaces.
'''
tcp_rcv_state_process()
  subflow_syn_recv_sock()
    tcp_init_transfer(BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB)
      bpf_skops_established       <== sockops
        bpf_sock_map_update(sk)   <== call bpf helper
          tcp_bpf_update_proto()  <== update sk_prot
'''
Consider two scenarios:

1. When the server has MPTCP enabled and the client also requests MPTCP,
   the sk passed to the BPF program is a subflow sk. Since subflows only
   handle partial data, replacing their sk_prot is meaningless and will
   cause traffic disruption.

2. When the server has MPTCP enabled but the client sends a TCP SYN
   without MPTCP, subflow_syn_recv_sock() performs a fallback on the
   subflow, replacing the subflow sk's sk_prot with the native sk_prot.
   '''
   subflow_ulp_fallback()
    subflow_drop_ctx()
      mptcp_subflow_ops_undo_override()
   '''
   Subsequently, accept::mptcp_stream_accept::mptcp_fallback_tcp_ops()
   converts the subflow to plain TCP.

For the first case, we should prevent it from being combined with sockmap
by setting sk_prot->psock_update_sk_prot to NULL, which will be blocked by
sockmap's own flow.

For the second case, since subflow_syn_recv_sock() has already restored
sk_prot to native tcp_prot/tcpv6_prot, no further action is needed.

Fixes: cec37a6e41aa ("mptcp: Handle MP_CAPABLE options for outgoing connections")
Signed-off-by: Jiayuan Chen <[email protected]>
Signed-off-by: Martin KaFai Lau <[email protected]>
Reviewed-by: Matthieu Baerts (NGI0) <[email protected]>
Cc: <[email protected]>
Link: https://patch.msgid.link/[email protected]

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