selftest: tcp: Fix address length in bind_wildcard.c.

The selftest passes the IPv6 address length for an IPv4 address.
We should pass the correct length.

Note inet_bind_sk() does not check if the size is larger than
sizeof(struct sockaddr_in), so there is no real bug in this
selftest.

Fixes: 13715acf

 ("selftest: Add test for bind() conflicts.")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>