dns-packet: move p->more unref into the free path

dns-packet: move p->more unref into the free path

dns_packet_unref() unconditionally unrefs p->more on every call,
even when n_ref > 1. But dns_packet_ref() doesn't ref p->more.
This means if a packet with a ->more chain gets ref'd and unref'd
multiple times, the chain gets freed too early while the parent
still holds a dangling pointer.

Move the p->more unref into the n_ref == 1 block so the chain
only gets cleaned up when the packet is actually being freed.