Found+fixed a bug where arbitrary dir seeks can return unrelated entries

It turned out our dir-read-idempotent test never created non-dstart
neighbors. This was a bit of a problem since we relied on dstart entries
to know when our dir read terminates. If we seek to an invalid position
(in theory undefined behavior, but easily possible with concurrent
modifications to the directory), we can end up reading an unrealted,
non-dstart entry, and incorrectly reporting that entry as in our current
dir.

This fix reintroduces the did into the lfsr_dir_t struct and uses the
did to determine end-of-dir. This adds some RAM cost, but is more
resilient to any seeks that overshoot the end of the directory.

Using did is also a stronger guarantee we will never accidentally report
unrelated entries as a part of the current directory.

lfs.h

@@ -424,6 +424,7 @@ typedef struct lfs_dir {
 
 typedef struct lfsr_dir {
     lfsr_openedmdir_t mdir;
+    lfs_size_t did;
     lfs_ssize_t dstart_mid;
     lfs_ssize_t dstart_rid;
     lfs_off_t pos;