v4l2bufferpool: cleanly handle streamon failure for output device

v4l2bufferpool: cleanly handle streamon failure for output device

On streamon failure, the queued buffer is not released from the
bufferpool class point of view because it is queued to the driver and
the flush logic is not performed since we are not in streaming state.
It causes the v4l2 bufferpool to always return that stop method failed
and to leak v4l2 objects and buffers.

This commit solve this by performing the flush logic in error case, ie
flushing the allocator and restoring queued buffer state to non-queued.

https://bugzilla.gnome.org/show_bug.cgi?id=738102