Drop `File_Len` from VFS.Read()

The `File_Len` is never supposed to change outside the FS driver.
diff --git a/src/filo-fs-ext2.adb b/src/filo-fs-ext2.adb
index ace8c6a..ec17fe4 100644
--- a/src/filo-fs-ext2.adb
+++ b/src/filo-fs-ext2.adb
@@ -751,7 +751,6 @@
 
    procedure Read
      (State    : in out T;
-      File_Len : in     File_Length;
       File_Pos : in out File_Offset;
       Buf      :    out Buffer_Type;
       Len      :    out Natural)
@@ -759,11 +758,11 @@
       Pos : Natural;
    begin
       if State.Inode.Mode = Fast_Link then
-         if File_Len > State.Inode.Inline'Length or
-            File_Pos >= File_Len then
+         if State.Inode.Size > Inode_Length (State.Inode.Inline'Length) or
+            Inode_Length (File_Pos) >= State.Inode.Size then
             Len := 0;
          else
-            Len := Natural'Min (Buf'Length, Natural (File_Len - File_Pos));
+            Len := Natural'Min (Buf'Length, Natural (State.Inode.Size) - Natural (File_Pos));
          end if;
          Buf (Buf'First .. Buf'First + Len - 1) :=
             State.Inode.Inline (Natural (File_Pos) .. Natural (File_Pos) + Len - 1);
@@ -780,13 +779,13 @@
             In_Block : constant Max_Block_Index := Natural (File_Pos mod Block_Size);
             Logical : constant FSBlock_Logical := FSBlock_Logical (File_Pos / Block_Size);
             In_Block_Space : constant Natural := Natural (Block_Size) - In_Block;
-            In_File_Space : constant Natural := Natural (File_Len - File_Pos);
+            In_File_Space : constant Inode_Length := State.Inode.Size - Inode_Length (File_Pos);
             In_Buf_Space : constant Natural := Buf'Last - Pos + 1;
             Len_Here : Natural;
          begin
             Len_Here := In_Block_Space;
-            if In_File_Space < Len_Here then
-               Len_Here := In_File_Space;
+            if In_File_Space < Inode_Length (Len_Here) then
+               Len_Here := Natural (In_File_Space);
             end if;
             if In_Buf_Space < Len_Here then
                Len_Here := In_Buf_Space;