BUG/MINOR: mux_quic: open an idle QCS on reset on BE side

BUG/MINOR: mux_quic: open an idle QCS on reset on BE side

On the backend side, a QCS may be opened but resetted immediately. No
STREAM frame will be emitted prior to the RESET_STREAM. When the latter
is sent, qcs_close_local() will mark the QCS Tx channel as closed.

In this case, a BUG_ON() would be triggered as there is QCS Tx channel
is not yet marked as opened. To prevent this, add a qcs_idle_open() call
when the stream is resetted, but only for the backend side.

This should be backported up to 3.3.