MFC: r234932

MFC: r234932

Added D_TRACKCLOSE to sndstat_cdevsw to fix the situation when
another process is in open() or stat() for the device node, then
close() from the owning process does not result in cdevsw close
method call. This fixes the pemanent "Device busy" seen.

Changed the sndstat_lock from mutex to sx.  This allows to extend
the region covered by the lock, to include the uiomove() call in
sndstat_read() and bufptr increment. This fixes the "panic:
sbuf_put_byte called with finished or corrupt sbuf" seen.

git-svn-id: svn://svn.freebsd.org/base/stable/8@235160 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f