MINOR: buffer: use b_orig() to replace most references to b->data

This patch updates most users of b->data to use b_orig().
diff --git a/include/common/buf.h b/include/common/buf.h
index b7a5df3..4632a33 100644
--- a/include/common/buf.h
+++ b/include/common/buf.h
@@ -364,7 +364,7 @@
 	if (l1 < max) {
 		*len1 = l1;
 		*len2 = max - l1;
-		*blk2 = buf->data;
+		*blk2 = b_orig(buf);
 		return 2;
 	}
 	*len1 = max;