mbuf: explicit cast of external buffer length

mbuf: explicit cast of external buffer length

GCC 8.1 warned:

rte_common.h:141:34:
warning: conversion from 'long unsigned int' to 'uint16_t'
{aka 'short unsigned int'} may change value [-Wconversion]
 #define RTE_PTR_DIFF(ptr1, ptr2) ((uintptr_t)(ptr1) - (uintptr_t)(ptr2))
                                  ^
rte_mbuf.h:1360:13:
note: in expansion of macro 'RTE_PTR_DIFF'
  *buf_len = RTE_PTR_DIFF(shinfo, buf_addr);

Fixes: a53aa2b9f3be ("mbuf: support attaching external buffer")

Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>