bpfilter: Initialize pos variable

Make sure 'pos' is initialized to zero before calling kernel_write().

Fixes: d2ba09c1 ("net: add skeleton of bpfilter kernel module")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/bpfilter/bpfilter_kern.c

@@ -48,6 +48,7 @@ static int __bpfilter_process_sockopt(struct sock *sk, int optname,
 	req.len = optlen;
 	if (!bpfilter_ops.info.tgid)
 		goto out;
+	pos = 0;
 	n = kernel_write(bpfilter_ops.info.pipe_to_umh, &req, sizeof(req),
 			   &pos);
 	if (n != sizeof(req)) {