ALSA: pcm: oss: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes, in particular in the
context in which this code is being used.

So, replace the following form:

sizeof(struct rate_priv) + src_format->channels * sizeof(struct rate_channel)

with:

struct_size(data, channels, src_format->channels)

This code was detected with the help of Coccinelle.
Signed-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/core/oss/rate.c

@@ -323,8 +323,8 @@ int snd_pcm_plugin_build_rate(struct snd_pcm_substream *plug,
 
 	err = snd_pcm_plugin_build(plug, "rate conversion",
 				   src_format, dst_format,
-				   sizeof(struct rate_priv) +
-				   src_format->channels * sizeof(struct rate_channel),
+				   struct_size(data, channels,
+					       src_format->channels),
 				   &plugin);
 	if (err < 0)
 		return err;