sctp: fix to only enable IPv6 address support on PF_INET6 socket

If socket is create by PF_INET type, it can not used IPv6 address
to send/recv DATA. So only enable IPv6 address support on PF_INET6
socket.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Signed-off-by: Vlad Yasevich <vladislav.yasevich@hp.com>