Services/AM: Support using FS subfiles with the CIA-related service functions.

FS subfiles are created with File::OpenSubFile, they have a start offset that must be added to all read/write operations.

The implementation in this commit is done using a new FileBackend that wraps the FS::File along with the start offset.

src/core/hle/service/fs/archive.cpp

@@ -307,6 +307,18 @@ Kernel::SharedPtr<Kernel::ClientSession> File::Connect() {
     return std::get<Kernel::SharedPtr<Kernel::ClientSession>>(sessions);
 }
 
+std::size_t File::GetSessionFileOffset(Kernel::SharedPtr<Kernel::ServerSession> session) {
+    const FileSessionSlot* slot = GetSessionData(session);
+    ASSERT(slot);
+    return slot->offset;
+}
+
+std::size_t File::GetSessionFileSize(Kernel::SharedPtr<Kernel::ServerSession> session) {
+    const FileSessionSlot* slot = GetSessionData(session);
+    ASSERT(slot);
+    return slot->size;
+}
+
 Directory::Directory(std::unique_ptr<FileSys::DirectoryBackend>&& backend,
                      const FileSys::Path& path)
     : ServiceFramework("", 1), path(path), backend(std::move(backend)) {

src/core/hle/service/fs/archive.h

@@ -73,6 +73,14 @@ public:
     /// Creates a new session to this File and returns the ClientSession part of the connection.
     Kernel::SharedPtr<Kernel::ClientSession> Connect();
 
+    // Returns the start offset of an open file represented by the input session, opened with
+    // OpenSubFile.
+    std::size_t GetSessionFileOffset(Kernel::SharedPtr<Kernel::ServerSession> session);
+
+    // Returns the size of an open file represented by the input session, opened with
+    // OpenSubFile.
+    std::size_t GetSessionFileSize(Kernel::SharedPtr<Kernel::ServerSession> session);
+
 private:
     void Read(Kernel::HLERequestContext& ctx);
     void Write(Kernel::HLERequestContext& ctx);