mptcp: more DATA FIN fixes

Currently data fin on data packet are not handled properly:
the 'rcv_data_fin_seq' field is interpreted as the last
sequence number carrying a valid data, but for data fin
packet with valid maps we currently store map_seq + map_len,
that is, the next value.

The 'write_seq' fields carries instead the value subseguent
to the last valid byte, so in mptcp_write_data_fin() we
never detect correctly the last DSS map.

Fixes: 7279da61 ("mptcp: Use MPTCP-level flag for sending DATA_FIN")
Fixes: 1a49b2c2 ("mptcp: Handle incoming 32-bit DATA_FIN values")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>