fs/ntfs3: allow readdir() to finish after directory mutations without rewinddir()

This patch introduces a per-directory version counter that increments on
each directory modification (indx_insert_entry() / indx_delete_entry()).
ntfs_readdir() uses this version to detect whether the directory has
changed since enumeration began. If readdir() reaches end-of-directory
but the version has changed, the walk restarts from the beginning of the
index tree instead of returning prematurely. This provides rmdir-like
behavior for tools that remove entries as they enumerate them.

Prior to this change, bonnie++ directory operations could fail due to
premature termination of readdir() during concurrent index updates.
With this patch applied, bonnie++ completes successfully with no errors.

Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>

fs/ntfs3/ntfs_fs.h

@@ -191,6 +191,7 @@ struct ntfs_index {
 	struct runs_tree alloc_run;
 	/* read/write access to 'bitmap_run'/'alloc_run' while ntfs_readdir */
 	struct rw_semaphore run_lock;
+	size_t version; /* increment each change */
 
 	/*TODO: Remove 'cmp'. */
 	NTFS_CMP_FUNC cmp;