BUG/MINOR: xprt_qstrm: prevent crash if conn release on MUX wake

BUG/MINOR: xprt_qstrm: prevent crash if conn release on MUX wake

When QMux XPRT has successfully been able to process to transport
parameters exchange, the MUX is initialized and immediately woken up to
start transfers. However, if the connection is in an unusable state, the
latter operation will instead release the connection and all of its
network stack.

A crash would occur in case of release when finalizing the XPRT tasklet
completion. To fix this, first free every XPRT resources. MUX wake is
now conducted in a safe way as the last operation before the tasklet is
completely released.

No need to backport.