BUG/MAJOR: quic: fix wrong packet building due to already acked frames

BUG/MAJOR: quic: fix wrong packet building due to already acked frames

If a packet build was asked to probe the peer with frames which have just
been acked, the frames build run by qc_build_frms() could be cancelled  by
qc_stream_frm_is_acked() whose aim is to check that current frames to
be built have not been already acknowledged. In this case the packet build run
by qc_do_build_pkt() is not interrupted, leading to the build of an empty packet
which should be ack-eliciting.

This is a bug detected by the BUG_ON() statement in qc_do_build_pk():

	    BUG_ON(qel->pktns->tx.pto_probe &&
           !(pkt->flags & QUIC_FL_TX_PACKET_ACK_ELICITING));

Thank you to @Tristan971 for having reported this issue in GH #2709

This is an old bug which must be backported as far as 2.6.