v4l2bufferpool: Acquire cannot return a buffer from another pool

v4l2bufferpool: Acquire cannot return a buffer from another pool

Return a buffer from an otherpool has unwanted side effects that lead to leaks and
prevents deactivating the pool. Instead, we change the _process() API so it can
replace the internal buffer with the buffer from the downstream pool. This implied
moving from _fill() to _create() method in the src.