mptcp: propagate sk_ipv6only to subflows

Usually, attributes are propagated to subflows as well.

Here, if subflows are created by other ways than the MPTCP path-manager,
it is important to make sure they are in v6 if it is asked by the
userspace.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>