mbuf: avoid integer promotion in prepend/adj/chain

mbuf: avoid integer promotion in prepend/adj/chain

GCC 8.1 warned:

In function 'rte_pktmbuf_prepend':
rte_mbuf.h:1908:17: warning: conversion from 'int' to 'uint16_t'
{aka 'short unsigned int'} may change value [-Wconversion]
  m->data_off -= len;
                 ^~~
m->data_off is a uint16_t

        uint16_t data_off;

len (a uint16_t) is promoted to an int using -=.  Do the
subtraction explicitly and cast the result to uint16_t.

The below += or -= changes are solving the same thing.

In function 'rte_pktmbuf_adj':
rte_mbuf.h:1969:17: warning: conversion from 'int' to 'uint16_t'
{aka 'short unsigned int'} may change value [-Wconversion]
  m->data_off += len;
                 ^~~

In function 'rte_pktmbuf_chain':
rte_mbuf.h:2082:19: warning: conversion from 'int' to 'uint16_t'
{aka 'short unsigned int'} may change value [-Wconversion]
  head->nb_segs += tail->nb_segs;
                   ^~~~
Also uint16_t

        uint16_t nb_segs;         /**< Number of segments. */

Fixes: 08b563ffb19d ("mbuf: replace data pointer by an offset")
Fixes: 1a60a0daa6e4 ("mbuf: fix segments number type increase")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>