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-cris/scatterlist.h b/include/asm-cris/scatterlist.h
index 5a8a834..faff53a 100644
--- a/include/asm-cris/scatterlist.h
+++ b/include/asm-cris/scatterlist.h
@@ -2,6 +2,9 @@
 #define __ASM_CRIS_SCATTERLIST_H
 
 struct scatterlist {
+#ifdef CONFIG_DEBUG_SG
+	unsigned long sg_magic;
+#endif
 	char *  address;    /* Location data is to be transferred to */
 	unsigned int length;