Some small tweaks to mdir comparison functions

- lfsr_mid_cmp no longer uses a union. This was undefined behavior and
  the lfsr_mid_t type isn't word aligned, so this could break pretty badly
  on machine/compiler change.

  Also dropped ordering based on endianness, since we need to marshal
  these into an int for the comparison anyways.

- Changed lfsr_mdir_cmp to use min/max functions as part of the
  comparison. The result is also "ordered" now, though the ordering
  is nonsensical. I guess the mrootanchor is less than all other mdirs?

  Also considered only comparing a single min/max block, since it would
  be an error for mdirs to share blocks, but note we rely on
  lfsr_mdir_cmp to check for relocations in lfsr_mdir_commit. These
  relocations can end up being partial in the case of bad block
  detection.

lfs.h

@@ -53,13 +53,8 @@ typedef uint16_t lfsr_mrid_t;
 typedef int16_t lfsr_smrid_t;
 
 typedef struct lfsr_mid {
-#ifdef LFS_BIG_ENDIAN
     lfsr_smbid_t bid;
     lfsr_smrid_t rid;
-#else
-    lfsr_smrid_t rid;
-    lfsr_smbid_t bid;
-#endif
 } lfsr_mid_t;
 
 // Maximum name size in bytes, may be redefined to reduce the size of the