dmix - Allow more flexible buffer sizes

dmix - Allow more flexible buffer sizes

With the patch, dmix allows apps to use more flexible buffer sizes.
The max buffer size is unlimited, and the minimal buffer size is
(period size * 2).  The buffer size is aligned to period size.

The period size is still bound to the period size of slave PCM.

To back to the old behavior (the fixed buffer size), you can set

   defaults.pcm.dmix_variable_buffer false

in your configuration.