ALSA: oss: Use flexible allocation for PCM plugins

Allocate PCM plugin objects with kzalloc_flex() for the trailing
extra data area instead of open-coding the size calculation.

This keeps the allocation tied to the existing flexible array member
without changing the plugin lifetime.

Assisted-by: Codex:GPT-5.5
Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://patch.msgid.link/20260519004647.627429-1-rosenp@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>

Author: neheb

sound/core/oss/pcm_plugin.c

@@ -146,7 +146,7 @@ int snd_pcm_plugin_build(struct snd_pcm_substream *plug,
 		return -ENXIO;
 	if (snd_BUG_ON(!src_format || !dst_format))
 		return -ENXIO;
-	plugin = kzalloc(sizeof(*plugin) + extra, GFP_KERNEL);
+	plugin = kzalloc_flex(*plugin, extra_data, extra);
 	if (plugin == NULL)
 		return -ENOMEM;
 	plugin->name = name;