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-s390/scatterlist.h b/include/asm-s390/scatterlist.h
index eb39486..29ec8e2 100644
--- a/include/asm-s390/scatterlist.h
+++ b/include/asm-s390/scatterlist.h
@@ -2,6 +2,9 @@
 #define _ASMS390_SCATTERLIST_H
 
 struct scatterlist {
+#ifdef CONFIG_DEBUG_SG
+    unsigned long sg_magic;
+#endif
     unsigned long page_link;
     unsigned int offset;
     unsigned int length;