rxrpc: Validate the net address given to rxrpc_kernel_begin_call()

Validate the net address given to rxrpc_kernel_begin_call() before using
it.

Whilst this should be mostly unnecessary for in-kernel users, it does clear
the tail of the address struct in case we want to hash or compare the whole
thing.
Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/af_rxrpc.c

@@ -280,9 +280,14 @@ struct rxrpc_call *rxrpc_kernel_begin_call(struct socket *sock,
 	struct rxrpc_transport *trans;
 	struct rxrpc_call *call;
 	struct rxrpc_sock *rx = rxrpc_sk(sock->sk);
+	int ret;
 
 	_enter(",,%x,%lx", key_serial(key), user_call_ID);
 
+	ret = rxrpc_validate_address(rx, srx, sizeof(*srx));
+	if (ret < 0)
+		return ERR_PTR(ret);
+
 	lock_sock(&rx->sk);
 
 	if (!key)