In cases where the number of tx rings is not a multiple of the number of
rx rings, the tx completion event will be handled on a different core
from the transmit and population of the ring. Races on the ring will
lead to a double-free of the page, and possibly other corruption.

The rings are initialized by default with a valid multiple of rings,
based on the number of cpus, therefore an invalid configuration requires
ethtool to change the ring layout. For instance 'ethtool -L eth0 rx 9 tx
8' will cause packets received on rx0, and XDP_TX'd to tx48, to be
completed on cpu3 (48 % 9 == 3).

Resolve this discrepancy by shifting the irq for the xdp tx queues to
start again from 0, modulo rx_ring_num.

Fixes: 9ecc2d86

 ("net/mlx4_en: add xdp forwarding and data write support")
Reported-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>
Signed-off-by: David S. Miller <davem@davemloft.net>