BUG/MINOR: mux_quic: prevent BE reuse with an errored conn

BUG/MINOR: mux_quic: prevent BE reuse with an errored conn

When a backend connection is reused, qcm_strm_attach() callback is used.
A BUG_ON() is present to ensure that the connection is not already on
error. This should be guaranteed by the fact that idle insertion is
skipped for such connections.

However, when a connection is flagged on error, it is not immediately
removed from its idle/avail pool. Thus, there is a risk that it is
reused, triggering the aformentioned BUG_ON() statement.

This issue should be avoided via avail_streams callback which should
return 0, forcing the caller to cancel the connection reuse. In QUIC,
this callback implementation relies on internal qcc_be_is_reusable().
However, it lacked checks for error status.

To fix this, extend qcc_be_is_reusable() to properly check connection
errors or an expired timeout.

Previously, these parameters were already checked by qcc_is_dead(). As
it also relies on qcc_be_is_reusable(), this patch also rearranges it to
avoid duplicate checks for backend connections.

This should be backported up to 3.3.