commit 6d29b1ef introduces a bug, ntohs is __be16_to_cpu,
not cpu_to_be16.

We always use htons on IP_OFFSET and IP_MF, then compare
with network package.
Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>