vfs: fix spurious error "vfs cache: failed to _ensure cache EOF"

Before this change the error message was produced for every file which
was confusing users.

After this change we check for EOF and return from ReadAt at that
point.

See: https://forum.rclone.org/t/rclone-1-53-release/18880/10

vfs/vfscache/item.go

@@ -1158,7 +1158,7 @@ func (item *Item) ReadAt(b []byte, off int64) (n int, err error) {
 		item.preAccess()
 		n, err = item.readAt(b, off)
 		item.postAccess()
-		if err == nil {
+		if err == nil || err == io.EOF {
 			break
 		}
 		fs.Errorf(item.name, "vfs cache: failed to _ensure cache %v", err)