Optimize the file iteration

Each `simplefs_extent` structure contains a counter that records the
total number of files within that extent. When the counter matches the
expected file number, it indicates there are no more files after this
index, allowing the iterator to skip directly to the next extension block.
This reduces unnecessary scanning and improves traversal efficiency.

Author: RoyWFHuang

dir.c

@@ -62,9 +62,10 @@ static int simplefs_iterate(struct file *dir, struct dir_context *ctx)
     for (; remained_nr_files && ei < SIMPLEFS_MAX_EXTENTS; ei++) {
         if (eblock->extents[ei].ee_start == 0)
             continue;
-
+        int ei_nr = eblock->extents[ei].nr_files;
         /* Iterate over blocks in one extent */
-        for (bi = 0; bi < eblock->extents[ei].ee_len && remained_nr_files;
+        for (bi = 0;
+             bi < eblock->extents[ei].ee_len && remained_nr_files && ei_nr;
              bi++) {
             bh2 = sb_bread(sb, eblock->extents[ei].ee_start + bi);
             if (!bh2) {
@@ -86,6 +87,7 @@ static int simplefs_iterate(struct file *dir, struct dir_context *ctx)
                         offset--;
                     } else {
                         remained_nr_files--;
+                        ei_nr--;
                         if (!dir_emit(ctx, dblock->files[fi].filename,
                                       SIMPLEFS_FILENAME_LEN,
                                       dblock->files[fi].inode, DT_UNKNOWN)) {