sound: Retire MIXER_SIZE and use correct size in DEFINE_CLASS

I am not sure why this hack was needed, but the size should be
sizeof(struct snd_mixer). Unfortunately, we have to expose snd_mixer,
but it should be better than relying on a hack.

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Pull Request:	https://ron-dev.freebsd.org/FreeBSD/src/pulls/18

sys/dev/sound/pcm/mixer.c

@@ -37,28 +37,6 @@
 #include "feeder_if.h"
 #include "mixer_if.h"
 
-#define MIXER_NAMELEN	16
-struct snd_mixer {
-	KOBJ_FIELDS;
-	void *devinfo;
-	int hwvol_mixer;
-	int hwvol_step;
-	int type;
-	device_t dev;
-	u_int32_t devs;
-	u_int32_t mutedevs;
-	u_int32_t recdevs;
-	u_int32_t recsrc;
-	u_int16_t level[32];
-	u_int16_t level_muted[32];
-	u_int8_t parent[32];
-	u_int32_t child[32];
-	u_int8_t realdev[32];
-	char name[MIXER_NAMELEN];
-	struct mtx lock;
-	int modify_counter;
-};
-
 static u_int16_t snd_mixerdefaults[SOUND_MIXER_NRDEVICES] = {
 	[SOUND_MIXER_VOLUME]	= 75,
 	[SOUND_MIXER_BASS]	= 50,

sys/dev/sound/pcm/mixer.h

@@ -30,6 +30,28 @@
 #ifndef _PCM_MIXER_H_
 #define	_PCM_MIXER_H_
 
+#define MIXER_NAMELEN	16
+struct snd_mixer {
+	KOBJ_FIELDS;
+	void *devinfo;
+	int hwvol_mixer;
+	int hwvol_step;
+	int type;
+	device_t dev;
+	u_int32_t devs;
+	u_int32_t mutedevs;
+	u_int32_t recdevs;
+	u_int32_t recsrc;
+	u_int16_t level[32];
+	u_int16_t level_muted[32];
+	u_int8_t parent[32];
+	u_int32_t child[32];
+	u_int8_t realdev[32];
+	char name[MIXER_NAMELEN];
+	struct mtx lock;
+	int modify_counter;
+};
+
 struct snd_mixer *mixer_create(device_t dev, kobj_class_t cls, void *devinfo,
     const char *desc);
 int mixer_delete(struct snd_mixer *m);
@@ -65,13 +87,7 @@ void *mix_getdevinfo(struct snd_mixer *m);
 #define MIXER_TYPE_PRIMARY	0	/* mixer_init()   */
 #define MIXER_TYPE_SECONDARY	1	/* mixer_create() */
 
-/*
- * this is a kludge to allow hiding of the struct snd_mixer definition
- * 512 should be enough for all architectures
- */
-#define MIXER_SIZE	(512 + sizeof(struct kobj) +		\
-			    sizeof(oss_mixer_enuminfo))
-
-#define MIXER_DECLARE(name) static DEFINE_CLASS(name, name ## _methods, MIXER_SIZE)
+#define MIXER_DECLARE(name) static DEFINE_CLASS(name, name ## _methods, \
+    sizeof(struct snd_mixer))
 
 #endif				/* _PCM_MIXER_H_ */