Restricted lfsr_ftree_t to ftree related things

Note, I think if we ever add file snapshots for idempotent errors again,
I don't think adding mdir/next back into the ftree is the best way to
structure this.

Instead, adding a separate linked-list for tracking bshrubs would work
without adding redundant mdir copies to the ftree struct.

Fortunately, in our current version, we don't need to track on-stack
ftrees. Actually, we don't make on-stack ftree copies at all...

lfs.h

@@ -537,8 +537,6 @@ 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;
@@ -551,6 +549,8 @@ 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;