tun: Fix copy/paste error in tun_get_user

Use the right structure while incrementing the offset in tun_get_user.
Signed-off-by: Sridhar Samudrala <sri@us.ibm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -571,7 +571,7 @@ static __inline__ ssize_t tun_get_user(struct tun_struct *tun,
 
 		if (gso.hdr_len > len)
 			return -EINVAL;
-		offset += sizeof(pi);
+		offset += sizeof(gso);
 	}
 
 	if ((tun->flags & TUN_TYPE_MASK) == TUN_TAP_DEV) {