tcp: consistently disable header prediction for mptcp

tcp: consistently disable header prediction for mptcp

The MPTCP receive path is hooked only into the TCP slow-path.
The DSS presence allows plain MPTCP traffic to hit that
consistently.

Since commit c5be4bc9a7f5 ("net: mptcp: improve fallback to TCP"),
when an MPTCP socket falls back to TCP, it can hit the TCP receive
fast-path, and delay or stop triggering the event notification.

Address the issue explicitly disabling the header prediction
for MPTCP sockets.

Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/200
Fixes: c5be4bc9a7f5 ("net: mptcp: improve fallback to TCP")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>