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-blackfin/scatterlist.h b/include/asm-blackfin/scatterlist.h
index 384af54..32128d5 100644
--- a/include/asm-blackfin/scatterlist.h
+++ b/include/asm-blackfin/scatterlist.h
@@ -4,6 +4,9 @@
 #include <linux/mm.h>
 
 struct scatterlist {
+#ifdef CONFIG_DEBUG_SG
+	unsigned long sg_magic;
+#endif
 	unsigned long page_link;
 	unsigned int offset;
 	dma_addr_t dma_address;