[NET] Generalise TCP's struct open_request minisock infrastructure

Kept this first changeset minimal, without changing existing names to
ease peer review.

Basicaly tcp_openreq_alloc now receives the or_calltable, that in turn
has two new members:

->slab, that replaces tcp_openreq_cachep
->obj_size, to inform the size of the openreq descendant for
  a specific protocol

The protocol specific fields in struct open_request were moved to a
class hierarchy, with the things that are common to all connection
oriented PF_INET protocols in struct inet_request_sock, the TCP ones
in tcp_request_sock, that is an inet_request_sock, that is an
open_request.

I.e. this uses the same approach used for the struct sock class
hierarchy, with sk_prot indicating if the protocol wants to use the
open_request infrastructure by filling in sk_prot->rsk_prot with an
or_calltable.

Results? Performance is improved and TCP v4 now uses only 64 bytes per
open request minisock, down from 96 without this patch :-)

Next changeset will rename some of the structs, fields and functions
mentioned above, struct or_calltable is way unclear, better name it
struct request_sock_ops, s/struct open_request/struct request_sock/g,
etc.

Signed-off-by: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/ip.h

@@ -81,6 +81,7 @@
 #ifdef __KERNEL__
 #include <linux/config.h>
 #include <linux/types.h>
+#include <net/request_sock.h>
 #include <net/sock.h>
 #include <linux/igmp.h>
 #include <net/flow.h>
@@ -107,6 +108,26 @@ struct ip_options {
 
 #define optlength(opt) (sizeof(struct ip_options) + opt->optlen)
 
+struct inet_request_sock {
+	struct open_request	req;
+	u32			loc_addr;
+	u32			rmt_addr;
+	u16			rmt_port;
+	u16			snd_wscale : 4, 
+				rcv_wscale : 4, 
+				tstamp_ok  : 1,
+				sack_ok	   : 1,
+				wscale_ok  : 1,
+				ecn_ok	   : 1,
+				acked	   : 1;
+	struct ip_options	*opt;
+};
+
+static inline struct inet_request_sock *inet_rsk(const struct open_request *sk)
+{
+	return (struct inet_request_sock *)sk;
+}
+
 struct ipv6_pinfo;
 
 struct inet_sock {