ALSA: pcm: Avoid possible info leaks from PCM stream buffers

ALSA: pcm: Avoid possible info leaks from PCM stream buffers

The current PCM code doesn't initialize explicitly the buffers
allocated for PCM streams, hence it might leak some uninitialized
kernel data or previous stream contents by mmapping or reading the
buffer before actually starting the stream.

Since this is a common problem, this patch simply adds the clearance
of the buffer data at hw_params callback.  Although this does only
zero-clear no matter which format is used, which doesn't mean the
silence for some formats, but it should be OK because the intention is
just to clear the previous data on the buffer.

Reported-by: Lionel Koenig <lionel.koenig@gmail.com>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191211155742.3213-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>