Add sg helpers for iterating over a scatterlist table

First step to being able to change the scatterlist setup without
having to modify drivers (a lot :-)
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

include/linux/scatterlist.h

@@ -20,4 +20,13 @@ static inline void sg_init_one(struct scatterlist *sg, const void *buf,
 	sg_set_buf(sg, buf, buflen);
 }
 
+#define sg_next(sg)		((sg) + 1)
+#define sg_last(sg, nents)	(&(sg[(nents) - 1]))
+
+/*
+ * Loop over each sg element, following the pointer to a new list if necessary
+ */
+#define for_each_sg(sglist, sg, nr, __i)	\
+	for (__i = 0, sg = (sglist); __i < (nr); __i++, sg = sg_next(sg))
+
 #endif /* _LINUX_SCATTERLIST_H */