media: videodev2.h: Add request_fd field to v4l2_buffer

When queuing buffers allow for passing the request that should
be associated with this buffer.

If V4L2_BUF_FLAG_REQUEST_FD is set, then request_fd is used as
the file descriptor.

If a buffer is stored in a request, but not yet queued to the
driver, then V4L2_BUF_FLAG_IN_REQUEST is set.

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>