net: add build-time checks for msg->msg_name size

This is a follow-up patch to f3d3342602f8bc ("net: rework recvmsg
handler msg_name and msg_namelen logic").

DECLARE_SOCKADDR validates that the structure we use for writing the
name information to is not larger than the buffer which is reserved
for msg->msg_name (which is 128 bytes). Also use DECLARE_SOCKADDR
consistently in sendmsg code paths.

Signed-off-by: Steffen Hurrle <steffen@hurrle.net>
Suggested-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/rxrpc/ar-output.c b/net/rxrpc/ar-output.c
index e1ac183..d0e8f1c 100644
--- a/net/rxrpc/ar-output.c
+++ b/net/rxrpc/ar-output.c
@@ -152,8 +152,8 @@
 	if (trans) {
 		service_id = rx->service_id;
 		if (msg->msg_name) {
-			struct sockaddr_rxrpc *srx =
-				(struct sockaddr_rxrpc *) msg->msg_name;
+			DECLARE_SOCKADDR(struct sockaddr_rxrpc *, srx,
+					 msg->msg_name);
 			service_id = htons(srx->srx_service);
 		}
 		key = rx->key;