mptcp: initialize the data_fin field for mpc packets

When parsing MPC+data packets we set the dss field, so
we must also initialize the data_fin, or we can find stray
value there.

Fixes: 9a19371b ("mptcp: fix data_fin handing in RX path")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>