Kernel/Devices: Introduce the LoopDevice device

This device is a block device that allows a user to effectively treat an
Inode as a block device.

The static construction method is given an OpenFileDescription reference
but validates that:
- The description has a valid custody (so it's not some arbitrary file).
  Failing this requirement will yield EINVAL.
- The description custody points to an Inode which is a regular file, as
  we only support (seekable) regular files. Failing this requirement
  will yield ENOTSUP.

LoopDevice can be used to mount a regular file on the filesystem like
other supported types of (physical) block devices.

Kernel/FileSystem/Ext2FS/FileSystem.h

@@ -47,6 +47,7 @@ public:
     virtual unsigned free_inode_count() const override;
 
     virtual bool supports_watchers() const override { return true; }
+    virtual bool supports_backing_loop_devices() const override { return true; }
 
     virtual u8 internal_file_type_to_directory_entry_type(DirectoryEntryView const& entry) const override;