net: constify VFTs

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/pppoe.c b/drivers/net/pppoe.c
index c22b305..5efc3d1 100644
--- a/drivers/net/pppoe.c
+++ b/drivers/net/pppoe.c
@@ -1030,7 +1030,7 @@
 	read_unlock_bh(&pppoe_hash_lock);
 }
 
-static struct seq_operations pppoe_seq_ops = {
+static const struct seq_operations pppoe_seq_ops = {
 	.start		= pppoe_seq_start,
 	.next		= pppoe_seq_next,
 	.stop		= pppoe_seq_stop,