dmm-buffer: check cache flush size
dmm-buffer: check cache flush size

Otherwise we might end up messing the whole process's memory.

Found by Mark Nauwelaerts.

Signed-off-by: Felipe Contreras <de77a8b9cd239b204c19c52ed62e48ab47243362@nokia.com>

community-ssu/gst-dsp,community-ssu/gst-dsp,felipec/gst-dsp,community-ssu/gst-dsp,felipec/gst-dsp