dmaengine: Constify struct dma_descriptor_metadata_ops

'struct dma_descriptor_metadata_ops' in not modified in these drivers.

Constifying these structures moves some data to a read-only section, so
increases overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
 120635	  21584	     64	 142283	  22bcb	drivers/dma/xilinx/xilinx_dma.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
 120699	  21520	     64	 142283	  22bcb	drivers/dma/xilinx/xilinx_dma.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Sai Sree Kartheek Adivi <s-adivi@ti.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Radhey Shyam Pandey <radhey.shyam.pandey@amd.com>
Link: https://patch.msgid.link/b0a22171f3ed68e156a2fa84383e99c23ec6b2ff.1784037977.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/ti/k3-udma.c

@@ -3408,7 +3408,7 @@ static int udma_set_metadata_len(struct dma_async_tx_descriptor *desc,
 	return 0;
 }
 
-static struct dma_descriptor_metadata_ops metadata_ops = {
+static const struct dma_descriptor_metadata_ops metadata_ops = {
 	.attach = udma_attach_metadata,
 	.get_ptr = udma_get_metadata_ptr,
 	.set_len = udma_set_metadata_len,

drivers/dma/xilinx/xilinx_dma.c

@@ -655,7 +655,7 @@ static void *xilinx_dma_get_metadata_ptr(struct dma_async_tx_descriptor *tx,
 	return seg->hw.app;
 }
 
-static struct dma_descriptor_metadata_ops xilinx_dma_metadata_ops = {
+static const struct dma_descriptor_metadata_ops xilinx_dma_metadata_ops = {
 	.get_ptr = xilinx_dma_get_metadata_ptr,
 };
 

include/linux/dmaengine.h

@@ -631,7 +631,7 @@ struct dma_async_tx_descriptor {
 	void *callback_param;
 	struct dmaengine_unmap_data *unmap;
 	enum dma_desc_metadata_mode desc_metadata_mode;
-	struct dma_descriptor_metadata_ops *metadata_ops;
+	const struct dma_descriptor_metadata_ops *metadata_ops;
 #ifdef CONFIG_ASYNC_TX_ENABLE_CHANNEL_SWITCH
 	struct dma_async_tx_descriptor *next;
 	struct dma_async_tx_descriptor *parent;