mptcp: support IPV6_V6ONLY setsockopt

mptcp: support IPV6_V6ONLY setsockopt

Without this, Opensshd fails to open an ipv6 socket listening
socket:
  error: setsockopt IPV6_V6ONLY: Operation not supported
  error: Bind to port 22 on :: failed: Address already in use.

Opensshd opens an ipv4 and and ipv6 listening socket, but because
IPV6_V6ONLY setsockopt fails, the port number is already in use.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>