Fix FAT/ext2 heap corruption: skip kfree on static root nodes

Fix FAT/ext2 heap corruption: skip kfree on static root nodes

fat_close_impl and ext2_close_impl unconditionally called kfree() on
the node passed to vfs_close(). When a directory fd pointed to a
filesystem mount root (a static variable like g_fat_root or
g_ext2_root), this kfree corrupted the heap (bad magic 0x0).

diskfs_close_impl already had the guard pattern (dn == &g_root).
Apply the same guard to fat and ext2 close handlers.