tracing: Don't make assumptions about length of string on task rename

While the dest comm string size is assured to be at least TASK_COMM_LEN long,
doing a memcpy() also adds the assumption that the source is at least that
long as well, which isn't assured, and isn't true in cases such as:

	set_task_comm(worker->task, "kworker/dying");

This leads to accessing invalid memory.

Link: http://lkml.kernel.org/r/1440760018-1557-1-git-send-email-sasha.levin@oracle.com

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Author: sashalevin

include/trace/events/task.h

@@ -46,7 +46,7 @@ TRACE_EVENT(task_rename,
 	TP_fast_assign(
 		__entry->pid = task->pid;
 		memcpy(entry->oldcomm, task->comm, TASK_COMM_LEN);
-		memcpy(entry->newcomm, comm, TASK_COMM_LEN);
+		strlcpy(entry->newcomm, comm, TASK_COMM_LEN);
 		__entry->oom_score_adj = task->signal->oom_score_adj;
 	),