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 cb510b8172602a66467f3551b4be1911f5a7c8c2
aka "seq_file: more atomicity in traverse()".

Signed-off-by: Alexey Dobriyan <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/seq_file.c

···
 		p = m->op->next(m, p, &index);
 	}
 	m->op->stop(m, p);
+	m->index = index;
 	return error;
 
 Eoverflow: