Kernel: Expose info about source devices of mounts in /proc/df

Author: https://github.com/bugaevc
Commit: fde8f7f538
Pull-request: https://github.com/SerenityOS/serenity/pull/456
Reviewed-by: https://github.com/awesomekling ✅

Kernel/FileSystem/FileSystem.h

@@ -65,6 +65,8 @@ public:
 
     int block_size() const { return m_block_size; }
 
+    virtual bool is_disk_backed() const { return false; }
+
 protected:
     FS();