tracing: Fix strncpy warning in trace_events_synth.c

0-day reported the strncpy error below:

../kernel/trace/trace_events_synth.c: In function 'last_cmd_set':
../kernel/trace/trace_events_synth.c:65:9: warning: 'strncpy' specified bound depends on the length o\
f the source argument [-Wstringop-truncation]
   65 |         strncpy(last_cmd, str, strlen(str) + 1);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../kernel/trace/trace_events_synth.c:65:32: note: length computed here
   65 |         strncpy(last_cmd, str, strlen(str) + 1);
      |                                ^~~~~~~~~~~

There's no reason to use strncpy here, in fact there's no reason to do
anything but a simple kstrdup() (note we don't even need to check for
failure since last_cmod is expected to be either the last cmd string
or NULL, and the containing function is a void return).

Link: https://lkml.kernel.org/r/77deca8cbfd226981b3f1eab203967381e9b5bd9.camel@kernel.org

Fixes: 27c888d ("tracing: Remove size restriction on synthetic event cmd error logging")

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Author: Tom Zanussi

kernel/trace/trace_events_synth.c

@@ -58,11 +58,8 @@ static void last_cmd_set(const char *str)
 		return;
 
 	kfree(last_cmd);
-	last_cmd = kzalloc(strlen(str) + 1, GFP_KERNEL);
-	if (!last_cmd)
-		return;
 
-	strncpy(last_cmd, str, strlen(str) + 1);
+	last_cmd = kstrdup(str, GFP_KERNEL);
 }
 
 static void synth_err(u8 err_type, u16 err_pos)