udmabuf: fix error code in map_udmabuf()

We accidentally forgot to set "ret" on this error path so it means we
return NULL instead of an error pointer.  The caller checks for NULL and
changes it to an error pointer so it doesn't cause an issue at run time.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20180914065615.GA12043@mwanda


Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>