repart: Use COPY_REFLINK in context_copy_blocks()

We might be copying between files without a loop device involved.
In that case, a reflink is possible and will be much faster.

src/partition/repart.c

@@ -3200,7 +3200,7 @@ static int context_copy_blocks(Context *context) {
                 log_info("Copying in '%s' (%s) on block level into future partition %" PRIu64 ".",
                          p->copy_blocks_path, FORMAT_BYTES(p->copy_blocks_size), p->partno);
 
-                r = copy_bytes(p->copy_blocks_fd, target_fd, p->copy_blocks_size, 0);
+                r = copy_bytes(p->copy_blocks_fd, target_fd, p->copy_blocks_size, COPY_REFLINK);
                 if (r < 0)
                         return log_error_errno(r, "Failed to copy in data from '%s': %m", p->copy_blocks_path);