Added directory list for synchronizing in flight directories

As it was, if a user operated on a directory while at the same
time iterating over the directory, the directory objects could
fall out of sync. In the best case, files may be skipped while
removing everything in a file, in the worst case, a very poorly
timed directory relocate could be missed.

Simple fix is to add the same directory tracking that is currently
in use for files, at a small code+complexity cost.

lfs.h

@@ -207,6 +207,7 @@ typedef struct lfs_file {
 } lfs_file_t;
 
 typedef struct lfs_dir {
+    struct lfs_dir *next;
     lfs_block_t pair[2];
     lfs_off_t off;
 
@@ -249,6 +250,7 @@ typedef struct lfs {
 
     lfs_block_t root[2];
     lfs_file_t *files;
+    lfs_dir_t *dirs;
 
     lfs_cache_t rcache;
     lfs_cache_t pcache;