udmabuf: actually unmap the scatterlist

unmap_udmabuf fails to actually unmap the scatterlist, leaving dangling
mappings around.

Fixes: fbb0de79

 ("Add udmabuf misc device")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Link: http://patchwork.freedesktop.org/patch/msgid/20190604202331.17482-1-l.stach@pengutronix.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>