In certain circumstances xn_txq_mq_start might be called with num_queues ==
0 during the resume phase after a migration, which can trigger a KASSERT.
Fix this by making sure the carrier is on before trying to transmit, or else
return that the queues are full.
Just as a note, I haven't been able to reproduce this crash on my test
systems, but I still think it's possible and worth fixing.
Reported by: Karl Pielorz <kpielorz_lst@tdx.co.uk>
Sponsored by: Citrix Systems R&D
MFC after: 1 week