seq_file: fix big-enough lseek() + read()

lseek() further than length of the file will leave stale ->index
(second-to-last during iteration). Next seq_read() will not notice
that ->f_pos is big enough to return 0, but will print last item
as if ->f_pos is pointing to it.

Introduced in commit cb510b81
aka "seq_file: more atomicity in traverse()".
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/seq_file.c

@@ -102,6 +102,7 @@ static int traverse(struct seq_file *m, loff_t offset)
 		p = m->op->next(m, p, &index);
 	}
 	m->op->stop(m, p);
+	m->index = index;
 	return error;
 
 Eoverflow: