rxrpc: Fix use-after-free in rxrpc_put_local()

Fix rxrpc_put_local() to not access local->debug_id after calling
atomic_dec_return() as, unless that returned n==0, we no longer have the
right to access the object.

Fixes: 06d9532 ("rxrpc: Fix read-after-free in rxrpc_queue_local()")
Signed-off-by: David Howells <dhowells@redhat.com>

net/rxrpc/local_object.c

@@ -364,11 +364,14 @@ void rxrpc_queue_local(struct rxrpc_local *local)
 void rxrpc_put_local(struct rxrpc_local *local)
 {
 	const void *here = __builtin_return_address(0);
+	unsigned int debug_id;
 	int n;
 
 	if (local) {
+		debug_id = local->debug_id;
+
 		n = atomic_dec_return(&local->usage);
-		trace_rxrpc_local(local->debug_id, rxrpc_local_put, n, here);
+		trace_rxrpc_local(debug_id, rxrpc_local_put, n, here);
 
 		if (n == 0)
 			call_rcu(&local->rcu, rxrpc_local_rcu);