udmabuf: rework limits

Create variable for the list length limit.  Serves as documentation,
also allows to make it a module parameter if needed.

Also add a total size limit.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20180911134216.9760-8-kraxel@redhat.com