btree: Renamed btree.leaf.rbyd -> btree.leaf.r

This matches other internal rbyds: btree.r, mdir.r, etc.

The intention of the single-char names is to reduce clutter around these
severely nested structs, both btrees and mdirs _are_ rbyds, so the name
doesn't really besides C-level type info.

I was hesitant on btree.leaf.rbyd, but decided consistency probably wins
here.

lfs3.h

@@ -691,7 +691,7 @@ typedef struct lfs3_btree {
     lfs3_rbyd_t r;
     struct {
         lfs3_bid_t bid;
-        lfs3_rbyd_t rbyd;
+        lfs3_rbyd_t r;
     } leaf;
 } lfs3_btree_t;