Kernel: Add a write cache to DiskBackedFS.

This way you can spam small write()s on a file without the kernel writing
to disk every single time. Flushes are included in the FS::sync() operation
and will get triggered regularly by syncd. :^)

Kernel/FileSystem/FileSystem.h

@@ -57,6 +57,8 @@ public:
 
     virtual RetainPtr<Inode> get_inode(InodeIdentifier) const = 0;
 
+    virtual void flush_writes() { }
+
 protected:
     FS();