v4l2src: only 1 bufferpool should be around, also during re-allocation
v4l2src: only 1 bufferpool should be around, also during re-allocation

... when so triggered by a downstream (post-tee) element.

commshare/gstreamer-imx,Josuercuevas/gstreamer-imx,commshare/gstreamer-imx,Josuercuevas/gstreamer-imx