[NET]: Size listen hash tables using backlog hint

We currently allocate a fixed size (TCP_SYNQ_HSIZE=512) slots hash table for
each LISTEN socket, regardless of various parameters (listen backlog for
example)

On x86_64, this means order-1 allocations (might fail), even for 'small'
sockets, expecting few connections. On the contrary, a huge server wanting a
backlog of 50000 is slowed down a bit because of this fixed limit.

This patch makes the sizing of listen hash table a dynamic parameter,
depending of :
- net.core.somaxconn tunable (default is 128)
- net.ipv4.tcp_max_syn_backlog tunable (default : 256, 1024 or 128)
- backlog value given by user application  (2nd parameter of listen())

For large allocations (bigger than PAGE_SIZE), we use vmalloc() instead of
kmalloc().

We still limit memory allocation with the two existing tunables (somaxconn &
tcp_max_syn_backlog). So for standard setups, this patch actually reduce RAM
usage.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/proto.c

@@ -262,12 +262,12 @@ int dccp_destroy_sock(struct sock *sk)
 
 EXPORT_SYMBOL_GPL(dccp_destroy_sock);
 
-static inline int dccp_listen_start(struct sock *sk)
+static inline int dccp_listen_start(struct sock *sk, int backlog)
 {
 	struct dccp_sock *dp = dccp_sk(sk);
 
 	dp->dccps_role = DCCP_ROLE_LISTEN;
-	return inet_csk_listen_start(sk, TCP_SYNQ_HSIZE);
+	return inet_csk_listen_start(sk, backlog);
 }
 
 int dccp_disconnect(struct sock *sk, int flags)
@@ -788,7 +788,7 @@ int inet_dccp_listen(struct socket *sock, int backlog)
 		 * FIXME: here it probably should be sk->sk_prot->listen_start
 		 * see tcp_listen_start
 		 */
-		err = dccp_listen_start(sk);
+		err = dccp_listen_start(sk, backlog);
 		if (err)
 			goto out;
 	}