resolve: llmnr: fix never hit condition

resolve: llmnr: fix never hit condition

Previously, the condition in on_stream_io_impl() never hit, as the
read packet is always taken from the stream in the few lines above.

Instead of the dns_stream_complete() under the condition, the stream
is unref()ed in the on_packet callback for LLMNR stream, unlike the
other on_packet callbacks.

That's quite tricky. Also, potentially, the stream may still have
queued packets to write.

This fix the condition, and drops the unref() in the on_packet callback.

C.f. https://github.com/systemd/systemd/pull/22274#issuecomment-1023708449.

Closes #22266.