media: videobuf2-dma-sg: fix vmap callback

commit 608ca5a6 upstream.

For dmabuf import users to be able to use the vaddr from another
videobuf2-dma-sg source, the exporter needs to set a proper vaddr on
vb2_dma_sg_dmabuf_ops_vmap callback. This patch adds vmap on map if
buf->vaddr was not set.

Cc: stable@kernel.org
Fixes: 7938f421 ("dma-buf-map: Rename to iosys-map")
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Acked-by: Tomasz Figa <tfiga@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>