pppoe: fix compiler warning

Fix warning introduced by 392fdb0 (net pppoe: Check packet length on all receive paths):

drivers/net/pppoe.c: In function 'pppoe_recvmsg':
drivers/net/pppoe.c:945: warning: comparison of distinct pointer types lacks a cast

Signed-off-by: Patrick McHardy <kaber@trash.net>
diff --git a/drivers/net/pppoe.c b/drivers/net/pppoe.c
index bafb69b..fc6f4b8 100644
--- a/drivers/net/pppoe.c
+++ b/drivers/net/pppoe.c
@@ -942,7 +942,7 @@ static int pppoe_recvmsg(struct kiocb *iocb, struct socket *sock,
 	m->msg_namelen = 0;
 
 	if (skb) {
-		total_len = min(total_len, skb->len);
+		total_len = min_t(size_t, total_len, skb->len);
 		error = skb_copy_datagram_iovec(skb, 0, m->msg_iov, total_len);
 		if (error == 0)
 			error = total_len;