ASoC: SOF: debug: Fix a potential issue on string buffer termination

The function simple_write_to_buffer() doesn't add string termination
at the end of buf, we need to handle it on our own. This change refers
to the function tokenize_input() in debug.c and the function
sof_dfsentry_trace_filter_write() in trace.c.

Fixes: 091c12e1 ("ASoC: SOF: debug: add new debugfs entries for IPC flood test")
Reviewed-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Signed-off-by: Hui Wang <hui.wang@canonical.com>
Link: https://lore.kernel.org/r/20210208103857.75705-1-hui.wang@canonical.comSigned-off-by: Mark Brown <broonie@kernel.org>

sound/soc/sof/debug.c

@@ -352,7 +352,7 @@ static ssize_t sof_dfsentry_write(struct file *file, const char __user *buffer,
 	char *string;
 	int ret;
 
-	string = kzalloc(count, GFP_KERNEL);
+	string = kzalloc(count+1, GFP_KERNEL);
 	if (!string)
 		return -ENOMEM;