In MPTCP, the receive windo is shared across all subflows, because it
refers to the mptcp-level sequence space.

This commit doesn't change choice of initial window for passive or active
connections.
While it would be possible to change those as well, this adds complexity
(especially when handling MP_JOIN requests).

However, the MPTCP RFC specifically says that a MPTCP sender
'MUST NOT use the RCV.WND field of a TCP segment at the connection level if
it does not also carry a DSS option with a Data ACK field.'

SYN/SYNACK packets do not carry a DSS option with a Data ACK field.

Signed-off-by: Florian Westphal <[email protected]>