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/remote/remote.go

@@ -113,6 +113,10 @@ func (bs *RemBackingStore) DataSync(offset, length int64) (err error) {
 }
 
 func (bs *RemBackingStore) Unmap(bd []api.UnmapBlockDescriptor) (err error) {
-	//_, err = bs.RemBs.Unmap(int64(bd[0].Offset), int64(bd[0].TL))
+	for _, desc := range bd {
+		if _, err = bs.RemBs.Unmap(int64(desc.Offset), int64(desc.TL)); err != nil {
+			return
+		}
+	}
 	return
 }