Kernel/FileSystem: Funnel calls to Inode::prepare_to_write_data method

Instead of requiring each FileSystem implementation to call this method
when trying to write data, do the calls at 2 points to avoid further
calls (or lack of them due to not remembering to use it) at other files
and locations in the codebase.

Kernel/FileSystem/Inode.h

@@ -31,6 +31,7 @@ class Inode : public ListedRefCounted<Inode, LockType::Spinlock>
     , public Weakable<Inode> {
     friend class VirtualFileSystem;
     friend class FileSystem;
+    friend class InodeFile;
 
 public:
     virtual ~Inode();