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/src/buffer.c b/src/buffer.c
index 16372bf..fe756f7 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -86,7 +86,7 @@
 
 	delta = len - (end - pos);
 
-	if (b_tail(b) + delta > b->data + b->size)
+	if (b_tail(b) + delta > b_wrap(b))
 		return 0;  /* no space left */
 
 	if (b_data(b) &&
@@ -123,7 +123,7 @@
 
 	delta = len + 2;
 
-	if (b_tail(b) + delta >= b->data + b->size)
+	if (b_tail(b) + delta >= b_wrap(b))
 		return 0;  /* no space left */
 
 	if (b_data(b) &&