netpoll: fix incorrect access to skb data in __netpoll_rx

__netpoll_rx() doesnt properly handle skbs with small header

pskb_may_pull() or pskb_trim_rcsum() can change skb->data, we must
reload it.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>