igb: cleanup igb_netpoll to be more friendly with napi & GRO

This patch cleans up igb_netpoll so that it is more friendly with both the
current napi and newly introduced GRO features.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>