selftests: net: reuseport_dualstack: fix uninitalized parameter
    
    This test reports EINVAL for getsockopt(SOL_SOCKET, SO_DOMAIN)
    occasionally due to the uninitialized length parameter.
    Initialize it to fix this, and also use int for "test_family" to comply
    with the API standard.
    
    Fixes: d6a61f80b871 ("soreuseport: test mixed v4/v6 sockets")
    Reported-by: Maciej Żenczykowski <maze@google.com>
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: Wei Wang <weiwan@google.com>
    Cc: Craig Gallek <cgallek@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>