mbuf: explicit cast of headroom on reset

mbuf: explicit cast of headroom on reset

GCC 8.1 warned:

rte_common.h:384:2:
warning: conversion from 'int' to 'uint16_t'
{aka 'short unsigned int'} may change value [-Wconversion]
  __extension__ ({ \
  ^~~~~~~~~~~~~
rte_mbuf.h:1204:16:
note: in expansion of macro 'RTE_MIN'
  m->data_off = RTE_MIN(RTE_PKTMBUF_HEADROOM, (uint16_t)m->buf_len);

RTE_PKTMBUF_HEADROOM is typ 128, so it doesn't make trouble.

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