t: Adopted simpler blanket-mdir traversal clobbering scheme

Now, lfsr_mdir_commit just clobbers all traversals associated with the
current mdir, irrespective of mid.

This makes our traversal clobbering model quite a bit simpler, drops any
mess related to bshrub staging, and allows lfsr_mdir_commit to handle
most of the clobbering logic with the exception of opened file handles.

This also fits mtree/mroot clobbering a bit better, with mtree
clobbering behaving the same as a file btree in the mroot.

The downside is we will miss more blocks during clobbered traversals,
but clobbered traversals are best effort anyways. The saved code cost
and simpler/more robust clobbering model are probably worth it.

Traversal clobbering is already complicated enough...

Code/stack changes:

           code          stack
  before: 34716           2648
  after:  34740 (+0.1%)   2624 (-0.9%)

lfs.h

@@ -610,7 +610,6 @@ typedef struct lfsr_mtraversal {
     // so only store the relevant bits, is this a hack? yes
     const struct lfs_file_config *cfg;
     lfsr_bshrub_t bshrub;
-    lfsr_bshrub_t bshrub_;
 
     // opened file state, we use an indirect pointer here so we
     // always point to data associated with the current mid