Ext2FS: Support shrinking inode to a smaller size.

Factor out inode resizing into a separate Ext2FSInode::resize() function.
This is then called both from write_bytes() and truncate().
This patch finally implements freeing of blocks when an inode shrinks.

Kernel/FileSystem/Ext2FileSystem.h

@@ -45,6 +45,7 @@ private:
     virtual KResult truncate(off_t) override;
 
     void populate_lookup_cache() const;
+    bool resize(qword);
 
     Ext2FS& fs();
     const Ext2FS& fs() const;