seq_open — initialize sequential file
int seq_open (struct file * file, struct seq_operations * op);
seq_open sets file, associating it with a sequence described
by op. op->start sets the iterator up and returns the first
element of sequence. op->stop shuts it down. op->next
returns the next element of sequence. op->show prints element
into the buffer. In case of error ->start and ->next return
ERR_PTR(error). In the end of sequence they return NULL. ->show
returns 0 in case of success and negative number in case of error.