udmabuf: set read/write flag when exporting

Otherwise, mmap fails when done with PROT_WRITE.
Suggested-by: NGurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>
Reviewed-by: NGurchetan Singh <gurchetansingh@chromium.org>
Tested-by: NGurchetan Singh <gurchetansingh@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20181114122029.16766-1-kraxel@redhat.com

drivers/dma-buf/udmabuf.c

@@ -184,6 +184,7 @@ static long udmabuf_create(const struct udmabuf_create_list *head,
 	exp_info.ops  = &udmabuf_ops;
 	exp_info.size = ubuf->pagecount << PAGE_SHIFT;
 	exp_info.priv = ubuf;
+	exp_info.flags = O_RDWR;
 
 	buf = dma_buf_export(&exp_info);
 	if (IS_ERR(buf)) {