net,socket: introduce DECLARE_SOCKADDR helper to catch overflow at build time

proto_ops->getname implies copying protocol specific data
into storage unit (particulary to __kernel_sockaddr_storage).
So when we implement new protocol support we should keep such
a detail in mind (which is easy to forget about).

Lets introduce DECLARE_SOCKADDR helper which check if
storage unit is not overfowed at build time.

Eventually inet_getname is switched to use DECLARE_SOCKADDR
(to show example of usage).

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/net.h

@@ -199,6 +199,9 @@ struct proto_ops {
 				       struct pipe_inode_info *pipe, size_t len, unsigned int flags);
 };
 
+#define DECLARE_SOCKADDR(type, dst, src)	\
+	type dst = ({ __sockaddr_check_size(sizeof(*dst)); (type) src; })
+
 struct net_proto_family {
 	int		family;
 	int		(*create)(struct net *net, struct socket *sock, int protocol);