sound: Add missing oss_mixerinfo devnode and legacy_device fields

They are missing from soundcard.h and are in fact used by some
applications, such as OSS' ossinfo(1):
http://manuals.opensound.com/developer/ossinfo.c.html

The new size for filler is chosen according to the most recent official
version of soundcard.h, which includes those 2 fields.

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Reviewed by:	dev_submerge.ch
Differential Revision:	https://reviews.freebsd.org/D45137

sys/dev/sound/pcm/mixer.c

@@ -1516,6 +1516,7 @@ mixer_oss_mixerinfo(struct cdev *i_dev, oss_mixerinfo *mi)
 			 * Mixer extensions currently aren't supported, so
 			 * leave @sa oss_mixerinfo::nrext blank for now.
 			 */
+
 			/**
 			 * @todo Fill in @sa oss_mixerinfo::priority (requires
 			 *       touching drivers?)
@@ -1529,13 +1530,11 @@ mixer_oss_mixerinfo(struct cdev *i_dev, oss_mixerinfo *mi)
 			 * default mixer. Other devices use values 1 to 9
 			 * depending on the estimated probability of being the
 			 * default device.
-			 *
-			 * XXX Described by Hannu@4Front, but not found in
-			 *     soundcard.h.
-			strlcpy(mi->devnode, devtoname(d->mixer_dev),
-			sizeof(mi->devnode));
-			mi->legacy_device = i;
 			 */
+
+			snprintf(mi->devnode, sizeof(mi->devnode), "/dev/mixer%d", i);
+			mi->legacy_device = i;
+
 			mtx_unlock(m->lock);
 		}
 

sys/sys/soundcard.h

@@ -1925,7 +1925,9 @@ typedef struct oss_mixerinfo
    * as the default mixer.
    */
   int priority;
-  int filler[254];		/* Reserved */
+  oss_devnode_t devnode;
+  int legacy_device;
+  int filler[245];		/* Reserved */
 } oss_mixerinfo;
 
 typedef struct oss_midi_info