t: Dropped const from btree/bshrub/mtree traverse functions

This could go either way, it's a case of the classic C strchr type
conundrum.

But unlike iteration, we're more likely to mutate things when doing a
full traversal, so requiring everything to be mutable makes a bit more
sense.

Note that even readonly operations, fetchck for example, need access to
a mutable rbyd struct.

No code changes:

           code          stack
  before: 35256           2680
  after:  35256 (+0.0%)   2680 (+0.0%)

lfs.h

@@ -615,7 +615,7 @@ typedef struct lfsr_dir {
 
 typedef struct lfsr_btraversal {
     lfsr_bid_t bid;
-    const lfsr_rbyd_t *branch;
+    lfsr_rbyd_t *branch;
     lfsr_srid_t rid;
     lfsr_rbyd_t rbyd;
 } lfsr_btraversal_t;