ixgbe: Use bool flag to see if the packet unmapping is delayed in HWRSC

We can't use zero magic "bad" value to check if IXGBE_RSC_CB(skb)->dma
is valid. It is only valid in x86/arm/m68k/alpha architectures and in
spark, powerPC and other architectures it should be ~0. As per
Benjamin Herrenschmidt feedback use a bool flag to decide if
the packet unmapping is delayed in hardware RSC till EOP is reached

Signed-off-by: Mallikarjuna R Chilakala <mallikarjuna.chilakala@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>