[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>

include/net/request_sock.h

@@ -28,8 +28,8 @@ struct proto;
 
 struct request_sock_ops {
 	int		family;
-	kmem_cache_t	*slab;
 	int		obj_size;
+	kmem_cache_t	*slab;
 	int		(*rtx_syn_ack)(struct sock *sk,
 				       struct request_sock *req,
 				       struct dst_entry *dst);
@@ -51,13 +51,13 @@ struct request_sock {
 	u32				rcv_wnd;	  /* rcv_wnd offered first time */
 	u32				ts_recent;
 	unsigned long			expires;
-	struct request_sock_ops		*rsk_ops;
+	const struct request_sock_ops	*rsk_ops;
 	struct sock			*sk;
 	u32				secid;
 	u32				peer_secid;
 };
 
-static inline struct request_sock *reqsk_alloc(struct request_sock_ops *ops)
+static inline struct request_sock *reqsk_alloc(const struct request_sock_ops *ops)
 {
 	struct request_sock *req = kmem_cache_alloc(ops->slab, SLAB_ATOMIC);
 
@@ -121,7 +121,7 @@ struct request_sock_queue {
 };
 
 extern int reqsk_queue_alloc(struct request_sock_queue *queue,
-			     const int nr_table_entries);
+			     unsigned int nr_table_entries);
 
 static inline struct listen_sock *reqsk_queue_yank_listen_sk(struct request_sock_queue *queue)
 {