tracing: Add 'hist:' to hist trigger error log error string

The 'hist:' prefix gets stripped from the command text during command
processing, but should be added back when displaying the command
during error processing.

Not only because it's what should be displayed but also because not
having it means the test cases fail because the caret is miscalculated
by the length of the prefix string.

Link: http://lkml.kernel.org/r/449df721f560042e22382f67574bcc5b4d830d3d.1561743018.git.zanussi@kernel.org

Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_events_hist.c

@@ -612,7 +612,8 @@ static void last_cmd_set(struct trace_event_file *file, char *str)
 	if (!str)
 		return;
 
-	strncpy(last_cmd, str, MAX_FILTER_STR_VAL - 1);
+	strcpy(last_cmd, "hist:");
+	strncat(last_cmd, str, MAX_FILTER_STR_VAL - 1 - sizeof("hist:"));
 
 	if (file) {
 		call = file->event_call;