iflib: netmap: honor netmap_irx_irq return values

[vmaffione]

Summary

In the receive interrupt routine, always call netmap_rx_irq(). The latter function will return != NM_IRQ_PASS if netmap is not active on that specific receive queue, so that the driver can go on with iflib_rxeof(). Note that netmap supports partial opening, where only a subset of the RX or TX rings can be open in netmap mode. Checking the IFCAP_NETMAP flag is not enough to make sure that the queue is indeed in netmap mode.
Moreover, in case netmap_rx_irq() returns NM_IRQ_RESCHED, it means that netmap expects the driver to call netmap_rx_irq() again as soon as possible. Currently, this may happen when the device is attached to a VALE switch.