FileSystem: Move block_size() from DiskBackedFS to FS

Let's just say that all filesystems have a block size, to keep things
nice and simple.

Kernel/FileSystem/DiskBackedFileSystem.cpp

@@ -139,14 +139,6 @@ ByteBuffer DiskBackedFS::read_blocks(unsigned index, unsigned count) const
     return blocks;
 }
 
-void DiskBackedFS::set_block_size(int block_size)
-{
-    ASSERT(block_size > 0);
-    if (block_size == m_block_size)
-        return;
-    m_block_size = block_size;
-}
-
 void DiskBackedFS::flush_writes()
 {
     LOCKER(m_lock);

Kernel/FileSystem/DiskBackedFileSystem.h

@@ -10,15 +10,11 @@ public:
     DiskDevice& device() { return *m_device; }
     const DiskDevice& device() const { return *m_device; }
 
-    int block_size() const { return m_block_size; }
-
     virtual void flush_writes() override;
 
 protected:
     explicit DiskBackedFS(NonnullRefPtr<DiskDevice>&&);
 
-    void set_block_size(int);
-
     ByteBuffer read_block(unsigned index) const;
     ByteBuffer read_blocks(unsigned index, unsigned count) const;
 
@@ -26,8 +22,6 @@ protected:
     bool write_blocks(unsigned index, unsigned count, const ByteBuffer&);
 
 private:
-    int m_block_size { 0 };
     NonnullRefPtr<DiskDevice> m_device;
-
     HashMap<unsigned, ByteBuffer> m_write_cache;
 };

Kernel/FileSystem/FileSystem.cpp

@@ -76,3 +76,10 @@ void FS::lock_all()
     }
 }
 
+void FS::set_block_size(int block_size)
+{
+    ASSERT(block_size > 0);
+    if (block_size == m_block_size)
+        return;
+    m_block_size = block_size;
+}

Kernel/FileSystem/FileSystem.h

@@ -61,13 +61,18 @@ public:
 
     virtual void flush_writes() {}
 
+    int block_size() const { return m_block_size; }
+
 protected:
     FS();
 
+    void set_block_size(int);
+
     mutable Lock m_lock { "FS" };
 
 private:
     unsigned m_fsid { 0 };
+    int m_block_size { 0 };
     bool m_readonly { false };
 };