mbuf: fix type of variables in linearize function

mbuf: fix type of variables in linearize function

GCC 8.1 warned:

In function 'rte_pktmbuf_linearize':
rte_mbuf.h:1873:32: warning: conversion to 'int' from 'uint32_t'
{aka 'unsigned int'} may change the sign of the result [-Wsign-conversion]
rte_mbuf.h:2166:13: note: in expansion of macro 'rte_pktmbuf_pkt_len'
   copy_len = rte_pktmbuf_pkt_len(mbuf) - rte_pktmbuf_data_len(mbuf);
rte_mbuf.h:2180:51: warning: conversion to 'size_t'
{aka 'long unsigned int'} from 'int' may change the
sign of the result [-Wsign-conversion]
rte_memcpy(buffer, rte_pktmbuf_mtod(m, char *), seg_len);
                                                ^~~~~~~

The temp is consumed as a size_t.  So let's make it
a size_t in the first place.

Fixes: 1feda4d8fc ("mbuf: add a function to linearize a packet")
Cc: stable@dpdk.org
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>