mptcp: drop all sub-options except ADD_ADDR when the echo bit is set

Current Linux carries echo-ed ADD_ADDR over pure TCP ACKs, so there is no
need to add a DSS element that would fit only ADD_ADDR with IPv4 address.
Drop the DSS from echo-ed ADD_ADDR, regardless of the IP version.

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