ipv4: add ip_sock_set_mtu_discover

Add a helper to directly set the IP_MTU_DISCOVER sockopt from kernel
space without going through a fake uaccess.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Howells <dhowells@redhat.com> [rxrpc bits]
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/rxrpc/output.c b/net/rxrpc/output.c
index f8b632a..1ba43c3 100644
--- a/net/rxrpc/output.c
+++ b/net/rxrpc/output.c
@@ -321,7 +321,7 @@ int rxrpc_send_data_packet(struct rxrpc_call *call, struct sk_buff *skb,
 	struct kvec iov[2];
 	rxrpc_serial_t serial;
 	size_t len;
-	int ret, opt;
+	int ret;
 
 	_enter(",{%d}", skb->len);
 
@@ -473,18 +473,14 @@ int rxrpc_send_data_packet(struct rxrpc_call *call, struct sk_buff *skb,
 	switch (conn->params.local->srx.transport.family) {
 	case AF_INET6:
 	case AF_INET:
-		opt = IP_PMTUDISC_DONT;
-		kernel_setsockopt(conn->params.local->socket,
-				  SOL_IP, IP_MTU_DISCOVER,
-				  (char *)&opt, sizeof(opt));
+		ip_sock_set_mtu_discover(conn->params.local->socket->sk,
+				IP_PMTUDISC_DONT);
 		ret = kernel_sendmsg(conn->params.local->socket, &msg,
 				     iov, 2, len);
 		conn->params.peer->last_tx_at = ktime_get_seconds();
 
-		opt = IP_PMTUDISC_DO;
-		kernel_setsockopt(conn->params.local->socket,
-				  SOL_IP, IP_MTU_DISCOVER,
-				  (char *)&opt, sizeof(opt));
+		ip_sock_set_mtu_discover(conn->params.local->socket->sk,
+				IP_PMTUDISC_DO);
 		break;
 
 	default: