mbuf: fix type of private size in detach

mbuf: fix type of private size in detach

GCC 8.1 warned:

In function 'rte_pktmbuf_detach':
rte_mbuf.h:1583:17: warning: conversion from 'uint32_t'
{aka 'unsigned int'} to 'uint16_t' {aka 'short unsigned int'}
may change value [-Wconversion]
  m->priv_size = priv_size;
                 ^~~~~~~~~

The temp priv_size is declared as a uint32_t.  But it
only deals in uint16_t.  m->priv_size is a uint16_t.
Change it to a uint16_t.

Fixes: 355e6735b3 ("mbuf: fix cloning with private mbuf data")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>