ixgbe: Set IXGBE_RSC_CB(skb)->DMA field to zero after unmapping the address

As per Simon Horman's feedback set IXGBE_RSC_CB(skb)->dma to zero
after unmapping HWRSC DMA address to avoid double freeing.

Signed-off-by: Mallikarjuna R Chilakala <mallikarjuna.chilakala@intel.com>
Acked-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>