mptcp: tune re-injections for csum enabled mode

If the MPTCP-level checksum is enabled, on re-injections we
must spool a complete DSS, or the receive side will not be
able to compute the csum and process any data.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>