net: fix skb_add_data_nocache() to calc csum correctly

commit c6e1a0d12ca7b4f22c58e55a16beacfb7d3d8462 broken the calc
 (net: Allow no-cache copy from user on transmit)
of checksum, which may cause some tcp packets be dropped because
incorrect checksum. ssh does not work under today's net-next-2.6
tree.

Signed-off-by: Wei Yongjun <yjwei@cn.fujitsu.com>
Acked-by: Tom Herbert <therbert@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>