net: explicit cast of IP checksum to 16-bit

net: explicit cast of IP checksum to 16-bit

GCC 8.1 warned:

In function 'rte_raw_cksum_mbuf':
rte_ip.h:225:22: warning: conversion from 'uint32_t'
{aka 'unsigned int'} to 'uint16_t' {aka 'short unsigned int'}
may change value [-Wconversion]
    tmp = rte_bswap16(tmp);
                      ^~~

In function 'rte_ipv4_cksum':
rte_ip.h:256:35: warning: conversion from 'int' to 'uint16_t'
{aka 'short unsigned int'} may change value [-Wconversion]
  return (cksum == 0xffff) ? cksum : ~cksum;
         ~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~

rte_ip.h:332:9: warning: conversion from 'uint32_t'
{aka 'unsigned int'} to 'uint16_t' {aka 'short unsigned int'}
may change value [-Wconversion]
  return cksum;
         ^~~~~

In function 'rte_ipv6_udptcp_cksum':
rte_ip.h:421:9: warning: conversion from 'uint32_t' {aka 'unsigned int'}
to 'uint16_t' {aka 'short unsigned int'} may change value [-Wconversion]
  return cksum;
         ^~~~~

Fixes: 6006818cfb26 ("net: new checksum functions")
Fixes: 4199fdea60c3 ("mbuf: generic support for TCP segmentation offload")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>