lksctp-tools: make bind_test can do while disable IPV6

When disable IPv6, the bind_test can't work for IPv4. So add
a variable 'domain' for support working for IPv4.

Signed-off-by: Wang Weidong <wangweidong1@huawei.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Vlad Yasevich <vyasevich@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>