PCM OSS layer may allocate a few temporary buffers, one for the core
read/write and another for the conversions via plugins.  Currently
both are allocated via vmalloc().  But as the allocation size is
equivalent with the PCM period size, the required size might be quite
small, depending on the application.

This patch replaces these vmalloc() calls with kvzalloc() for covering
small period sizes better.  Also, we use "z"-alloc variant here for
addressing the possible uninitialized access reported by syzkaller.

Reported-by:  <syzbot+1cb36954e127c98dd037@syzkaller.appspotmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>