Kernel: Expose FileSystem's fragment size

This commit will add a fragment_size() function similar to the
block_size() function.

Kernel/FileSystem/FileSystem.cpp

@@ -81,4 +81,12 @@ void FS::set_block_size(size_t block_size)
     m_block_size = block_size;
 }
 
+void FS::set_fragment_size(size_t fragment_size)
+{
+    VERIFY(fragment_size > 0);
+    if (fragment_size == m_fragment_size)
+        return;
+    m_fragment_size = fragment_size;
+}
+
 }