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/DiskBackedFileSystem.h

@@ -7,6 +7,8 @@ class DiskBackedFS : public FS {
 public:
     virtual ~DiskBackedFS() override;
 
+    virtual bool is_disk_backed() const override { return true; }
+
     DiskDevice& device() { return *m_device; }
     const DiskDevice& device() const { return *m_device; }