Kernel: Add InodeFile, a File subclass for regular files.

Finally everything that can be held by a FileDescriptor actually inherits
from the File class.

Kernel/FileSystem/InodeFile.h

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <Kernel/File.h>
+
+class Inode;
+
+class InodeFile final : public File {
+public:
+    static Retained<InodeFile> create(Retained<Inode>&& inode)
+    {
+        return adopt(*new InodeFile(move(inode)));
+    }
+
+    virtual ~InodeFile() override;
+
+    const Inode& inode() const { return *m_inode; }
+    Inode& inode() { return *m_inode; }
+
+    virtual bool can_read(FileDescriptor&) const override { return true; }
+    virtual bool can_write(FileDescriptor&) const override { return true; }
+
+    virtual ssize_t read(FileDescriptor&, byte*, ssize_t) override;
+    virtual ssize_t write(FileDescriptor&, const byte*, ssize_t) override;
+    virtual KResultOr<Region*> mmap(Process&, LinearAddress preferred_laddr, size_t offset, size_t size, int prot) override;
+
+    virtual String absolute_path(FileDescriptor&) const override;
+
+    virtual KResult truncate(off_t) override;
+
+    virtual const char* class_name() const override { return "InodeFile"; }
+
+    virtual bool is_seekable() const override { return true; }
+    virtual bool is_inode() const override { return true; }
+
+private:
+    explicit InodeFile(Retained<Inode>&&);
+    Retained<Inode> m_inode;
+};