Add CONFIG_DEBUG_SG sg validation

Add a Kconfig entry which will toggle some sanity checks on the sg
entry and tables.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/include/asm-sh/scatterlist.h b/include/asm-sh/scatterlist.h
index bc7c809..a7d0d18 100644
--- a/include/asm-sh/scatterlist.h
+++ b/include/asm-sh/scatterlist.h
@@ -4,6 +4,9 @@
 #include <asm/types.h>
 
 struct scatterlist {
+#ifdef CONFIG_DEBUG_SG
+    unsigned long sg_magic;
+#endif
     unsigned long page_link;
     unsigned int offset;/* for highmem, page offset */
     dma_addr_t dma_address;