The combination of supports_requests == 1 and min_buffers_needed > 0
is not allowed, WARN on that and return an error.

Also check that if vb2_core_qbuf() is called from req_queue, that it
doesn't return an error, unless it is -EIO.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>