netpoll: fix position of network header

Similar to the problem in pktgen, netpoll uses skb_tail_offset()
too, as the code is copied from pktgen.

Also use return values of skb_put() directly, this will simiplify
the code.

Reported-by: Thomas Graf <tgraf@suug.ch>
Cc: Thomas Graf <tgraf@suug.ch>
Cc: Daniel Borkmann <dborkmann@redhat.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>