fix UNMAP data corruption by implementing block zeroing

The UNMAP command was a no-op in all backing stores, causing unmapped
blocks to retain stale data instead of returning zeros per SCSI spec.

- Implement Unmap in FileBackingStore to zero out unmapped blocks
- Implement Unmap in IOUringBackingStore to zero out unmapped blocks
- Enable Unmap in RemBackingStore (was commented out)
- Change UnmapBlockDescriptor.TL from uint32 to uint64 to prevent
  integer overflow when converting block count to byte length with
  large block shifts

Fixes #119

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

pkg/scsi/backingstore/common.go

@@ -159,6 +159,12 @@ func (bs *FileBackingStore) DataAdvise(offset, length int64, advise uint32) erro
 	return util.Fadvise(bs.file, offset, length, advise)
 }
 
-func (bs *FileBackingStore) Unmap([]api.UnmapBlockDescriptor) error {
+func (bs *FileBackingStore) Unmap(descriptors []api.UnmapBlockDescriptor) error {
+	for _, desc := range descriptors {
+		zeros := make([]byte, desc.TL)
+		if _, err := bs.file.WriteAt(zeros, int64(desc.Offset)); err != nil {
+			return err
+		}
+	}
 	return nil
 }