net: constify VFTs

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/pppoe.c

@@ -1030,7 +1030,7 @@ static void pppoe_seq_stop(struct seq_file *seq, void *v)
 	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,