net: add sock_set_keepalive

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

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sunrpc/xprtsock.c

@@ -2110,7 +2110,6 @@ static void xs_tcp_set_socket_timeouts(struct rpc_xprt *xprt,
 	struct sock_xprt *transport = container_of(xprt, struct sock_xprt, xprt);
 	unsigned int keepidle;
 	unsigned int keepcnt;
-	unsigned int opt_on = 1;
 	unsigned int timeo;
 
 	spin_lock(&xprt->transport_lock);
@@ -2122,8 +2121,7 @@ static void xs_tcp_set_socket_timeouts(struct rpc_xprt *xprt,
 	spin_unlock(&xprt->transport_lock);
 
 	/* TCP Keepalive options */
-	kernel_setsockopt(sock, SOL_SOCKET, SO_KEEPALIVE,
-			(char *)&opt_on, sizeof(opt_on));
+	sock_set_keepalive(sock->sk);
 	kernel_setsockopt(sock, SOL_TCP, TCP_KEEPIDLE,
 			(char *)&keepidle, sizeof(keepidle));
 	kernel_setsockopt(sock, SOL_TCP, TCP_KEEPINTVL,