Check for held buffers that are ready to be returned to vb2 in
__v4l2_m2m_try_queue(). This avoids drivers having to handle this
case.

Add v4l2_m2m_buf_done_and_job_finish() to correctly return source
and destination buffers and mark the job as finished while taking
a held destination buffer into account (i.e. that buffer won't be
returned). This has to be done while job_spinlock is held to avoid
race conditions.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>