enic: alloc/free rx_cpu_rmap

rx_cpu_rmap provides the reverse irq cpu affinity. This patch allocates and
sets drivers netdev->rx_cpu_rmap accordingly.

rx_cpu_rmap is set in enic_request_intr() which is called by enic_open and
rx_cpu_rmap is freed in enic_free_intr() which is called by enic_stop.

This is used by Accelerated RFS.

Signed-off-by: Govindarajulu Varadarajan <_govind@gmx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>