jbd2_seq_info_next should increase position index

commit 1a8e9cf4 upstream.

if seq_file .next fuction does not change position index,
read after some lseek can generate unexpected output.

Script below generates endless output
 $ q=;while read -r r;do echo "$((++q)) $r";done </proc/fs/jbd2/DEV/info

https://bugzilla.kernel.org/show_bug.cgi?id=206283

Fixes: 1f4aace6 ("fs/seq_file.c: simplify seq_file iteration code and interface")
Cc: stable@kernel.org
Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/d13805e5-695e-8ac3-b678-26ca2313629f@virtuozzo.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/jbd2/journal.c

@@ -1002,6 +1002,7 @@ static void *jbd2_seq_info_start(struct seq_file *seq, loff_t *pos)
 
 static void *jbd2_seq_info_next(struct seq_file *seq, void *v, loff_t *pos)
 {
+	(*pos)++;
 	return NULL;
 }