net: Use helper macro IP_MAX_MTU in __ip_append_data()

What 0xFFFF means here is actually the max mtu of a ip packet. Use help
macro IP_MAX_MTU here.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>