mptcp: Implement MPTCP receive path

Parses incoming DSS options and populates outgoing MPTCP ACK
fields. MPTCP fields are parsed from the TCP option header and placed in
an skb extension, allowing the upper MPTCP layer to access MPTCP
options after the skb has gone through the TCP stack.

The subflow implements its own data_ready() ops, which ensures that the
pending data is in sequence - according to MPTCP seq number - dropping
out-of-seq skbs. The DATA_READY bit flag is set if this is the case.
This allows the MPTCP socket layer to determine if more data is
available without having to consult the individual subflows.

It additionally validates the current mapping and propagates EoF events
to the connection socket.

Co-developed-by: Paolo Abeni <[email protected]>
Signed-off-by: Paolo Abeni <[email protected]>
Co-developed-by: Peter Krystad <[email protected]>
Signed-off-by: Peter Krystad <[email protected]>
Co-developed-by: Davide Caratti <[email protected]>
Signed-off-by: Davide Caratti <[email protected]>
Co-developed-by: Matthieu Baerts <[email protected]>
Signed-off-by: Matthieu Baerts <[email protected]>
Co-developed-by: Florian Westphal <[email protected]>
Signed-off-by: Florian Westphal <[email protected]>
Signed-off-by: Mat Martineau <[email protected]>
Signed-off-by: Christoph Paasch <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

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