Extended lfsr_ftree_t to include the opened-mdir

Now that bshrubs are limited to files, we might as well lean into it.

Instead of relying on extra traversals through our attr-list, we now
rely on bsprouts/bshrubs always existing in the opened mdir list. This
is a much more robust way to deduplicate bsprout/bshrub compaction
operations. The previous implementation already had a bug since
bsprout/bshrub calculation was forgotten from lfsr_mdir_estimate_.

This is a direct tradeoff of code and RAM. Though note the previous
impl was incomplete and would likely need a bit more code:

            code          stack
  before:  33184           2944
  after:   32980 (-0.6%)   3000 (+1.9%)

lfs.h

@@ -535,6 +535,8 @@ typedef struct lfsr_bshrub {
 
 // the lfsr_ftree_t struct is a sort of proto-file
 typedef struct lfsr_ftree {
+    lfsr_openedmdir_t *next;
+    lfsr_mdir_t mdir;
     union {
         lfs_soff_t size;
         lfsr_data_t data;
@@ -547,8 +549,7 @@ typedef struct lfsr_ftree {
 } lfsr_ftree_t;
 
 typedef struct lfsr_file {
-    lfsr_openedmdir_t *next;
-    lfsr_mdir_t mdir;
+    lfsr_ftree_t ftree;
     uint32_t flags;
     lfs_off_t pos;
     lfs_off_t size;
@@ -557,8 +558,6 @@ typedef struct lfsr_file {
     uint8_t *buffer;
     lfs_size_t buffer_size;
 
-    lfsr_ftree_t ftree;
-
     const struct lfs_file_config *cfg;
 } lfsr_file_t;