t: Changed lfsr_mtree_traverse to operate on mdir+mtraversal

Separated out omdir/mdir and mtraversal. You still need to allocate an
mdir for mtraversal to work, but this avoids the extra cost of omdir's
linked-list.

To avoid _too_ many pointers, I duplicated the flags field into both
lfsr_traversal_t and lfsr_mtraversal_t. This is basically free since we
end up with a bunch of padding for mtraversal's state field, but comes
with the risk of getting confused when the two flag fields don't match
in the future.

I also merged the intermediary btype field into flags to avoid yet
another single-byte field, where it fits comfortably in 3-bits.

Note that the mdir can be uninitialized in cases where we don't need to
worry about traversal clobbering.

---

This has the same problems as separating out mdirs/bshrubs in bshrub
functions: more stack/code to move the multiple pointers around, but is
necessary to avoid strict aliasing issues. There's no way to represent
overlapping omdir/mdir/mtraversal struct in standard C99 otherwise.

The end result saves a bit of code, but adds a bit of stack:

           code          stack
  before: 34576           2632
  after:  34524 (-0.2%)   2640 (+0.3%)

Though these numbers may be close enough to the compiler noise floor to
not really care about...

lfs.h

@@ -474,7 +474,6 @@ typedef struct lfsr_mdir {
 typedef struct lfsr_omdir {
     struct lfsr_omdir *next;
     uint8_t type;
-    uint8_t state;
     uint16_t flags;
     lfsr_mdir_t mdir;
 } lfsr_omdir_t;
@@ -603,10 +602,11 @@ typedef struct lfsr_btraversal {
 } lfsr_btraversal_t;
 
 typedef struct lfsr_mtraversal {
-    // core state machine in o.state
-    lfsr_omdir_t o;
+    // core state machine
+    uint8_t state;
+    uint16_t flags;
     // opened file state
-    lfsr_omdir_t *ot;
+    lfsr_omdir_t *o;
     // bshrub/btree state
     lfsr_bshrub_t bshrub;
     union {
@@ -622,9 +622,8 @@ typedef struct lfsr_mtraversal {
 } lfsr_mtraversal_t;
 
 typedef struct lfsr_traversal {
-    // lfsr_mtraversal_t contains most of what we need
+    lfsr_omdir_t o;
     lfsr_mtraversal_t mt;
-    uint8_t btype;
     lfs_sblock_t blocks[2];
 } lfsr_traversal_t;