fat: Move struct fat_dir to top of file

Move struct definitions to the top of the file for better organization
and visibility.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

fs/fat/fat.c

@@ -27,6 +27,13 @@
 #include <linux/log2.h>
 #include "fat_internal.h"
 
+struct fat_dir {
+	struct fs_dir_stream parent;
+	struct fs_dirent dirent;
+	struct fsdata fsdata;
+	struct fat_itr itr;
+};
+
 /*
  * Convert a string to lowercase.  Converts at most 'len' characters,
  * 'len' may be larger than the length of 'str' if 'str' is NULL
@@ -1190,13 +1197,6 @@ int file_fat_read(const char *filename, void *buffer, int maxsize)
 		return actread;
 }
 
-struct fat_dir {
-	struct fs_dir_stream parent;
-	struct fs_dirent dirent;
-	struct fsdata fsdata;
-	struct fat_itr itr;
-};
-
 int fat_opendir(const char *filename, struct fs_dir_stream **dirsp)
 {
 	struct fat_dir *dir;