seq_tab_next() should increase position index

BugLink: https://bugs.launchpad.net/bugs/1864774

[ Upstream commit 70a87287 ]

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

https://bugzilla.kernel.org/show_bug.cgi?id=206283Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/net/ethernet/chelsio/cxgb4/cxgb4_debugfs.c

@@ -66,8 +66,7 @@ static void *seq_tab_start(struct seq_file *seq, loff_t *pos)
 static void *seq_tab_next(struct seq_file *seq, void *v, loff_t *pos)
 {
 	v = seq_tab_get_idx(seq->private, *pos + 1);
-	if (v)
-		++*pos;
+	++(*pos);
 	return v;
 }