media: core: Free range of buffers

Improve __vb2_queue_free() and __vb2_free_mem() to free
range of buffers and not only the last few buffers.
Introduce starting index to be flexible on range and change the loops
according to this parameter.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Acked-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>