[Fix] Preserve req->pos during reply validation to prevent packet truncation on retra...

[Fix] Preserve req->pos during reply validation to prevent packet truncation on retransmit

The rdns_request_reply_cmp function modifies req->pos as a side effect
during reply validation. This caused packet truncation on retransmits
because req->pos (which tracks the full packet length) was overwritten
with the end position of the question section.

On timeout/retry, rdns_send_request would use the corrupted req->pos
value, resulting in truncated packets missing the OPT additional section.
This made resolvers like Knot and PowerDNS unable to parse retransmitted
packets.

Fix by saving and restoring req->pos around the reply comparison logic.