dma-buf: Adjust a null pointer check in dma_buf_attach()

The script "checkpatch.pl" pointed information out like the following.

Comparison to NULL could be written "!attach"

Thus adjust this expression.
Signed-off-by: NMarkus Elfring <elfring@users.sourceforge.net>
Reviewed-by: NGustavo Padovan <gustavo.padovan@collabora.com>
Signed-off-by: NSumit Semwal <sumit.semwal@linaro.org>

drivers/dma-buf/dma-buf.c

@@ -559,7 +559,7 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
 		return ERR_PTR(-EINVAL);
 
 	attach = kzalloc(sizeof(*attach), GFP_KERNEL);
-	if (attach == NULL)
+	if (!attach)
 		return ERR_PTR(-ENOMEM);
 
 	attach->dev = dev;