net/bpfilter: Initialize pos in __bpfilter_process_sockopt

__bpfilter_process_sockopt never initialized the pos variable passed
to the pipe write. This has been mostly harmless in the past as pipes
ignore the offset, but the switch to kernel_write now verified the
position, which can lead to a failure depending on the exact stack
initialization pattern. Initialize the variable to zero to make
rw_verify_area happy.

Fixes: 6955a76f ("bpfilter: switch to kernel_write")
Reported-by: Christian Brauner <christian.brauner@ubuntu.com>
Reported-by: Rodrigo Madera <rodrigo.madera@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Rodrigo Madera <rodrigo.madera@gmail.com>
Tested-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/bpf/20200730160900.187157-1-hch@lst.de

net/bpfilter/bpfilter_kern.c

@@ -39,7 +39,7 @@ static int __bpfilter_process_sockopt(struct sock *sk, int optname,
 {
 	struct mbox_request req;
 	struct mbox_reply reply;
-	loff_t pos;
+	loff_t pos = 0;
 	ssize_t n;
 	int ret = -EFAULT;