mptcp: do not set unconditionally csum_reqd on incoming opt
Should be set only if the ingress packets present it, otherwise we can confuse csum validation. 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>