cobalt/rtdm: Fix tracing of failed fd operation

Also if some fd operation failed, we trace the result via the
fd_request_status class. However, fd will then contain an error code,
not a valid pointer. Avoid dereferencing such invalid pointers and
causing an oops this way.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kernel/cobalt/trace/cobalt-rtdm.h

@@ -75,7 +75,8 @@ DECLARE_EVENT_CLASS(fd_request_status,
 	TP_fast_assign(
 		memcpy(__entry->comm, task->comm, TASK_COMM_LEN);
 		__entry->pid = task->pid;
-		__entry->dev	= rtdm_fd_to_context(fd)->device;
+		__entry->dev =
+			!IS_ERR(fd) ? rtdm_fd_to_context(fd)->device : NULL;
 		__entry->ufd = ufd;
 	),