media: vb2: add init_buffer buffer op

We need to initialize the request_fd field in struct vb2_v4l2_buffer
to -1 instead of the default of 0. So we need to add a new op that
is called when struct vb2_v4l2_buffer is allocated.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>