Kernel: Allow SysFS components to have non-zero size

This is important for dmidecode because it does an fstat on the DMI
blobs, trying to figure out their size. Because we already know the size
of the blobs when creating the SysFS components, there's no performance
penalty whatsoever, and this allows dmidecode to not use the /dev/mem
device as a fallback.

Kernel/FileSystem/SysFSComponent.h

@@ -31,6 +31,7 @@ public:
     virtual RefPtr<SysFSComponent> lookup(StringView) { VERIFY_NOT_REACHED(); };
     virtual mode_t permissions() const;
     virtual ErrorOr<void> truncate(u64) { return EPERM; }
+    virtual size_t size() const { return 0; }
     virtual ErrorOr<void> set_mtime(time_t) { return ENOTIMPL; }
     virtual ErrorOr<size_t> write_bytes(off_t, size_t, UserOrKernelBuffer const&, OpenFileDescription*) { return EROFS; }
     virtual ErrorOr<void> refresh_data(OpenFileDescription&) const { return {}; }