ptnetmap: deny passthrough of emulated adapter with ptnetmap_tx_workers == 0
ptnetmap: deny passthrough of emulated adapter with ptnetmap_tx_workers == 0

This is necessary because dev_queue_xmit() cannot be called under
spinlock.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap