eal/x86: fix type of variable in memcpy function

eal/x86: fix type of variable in memcpy function

GCC 8.1 warned:

rte_memcpy.h:793:2: note: in expansion of macro 'MOVEUNALIGNED_LEFT47'
  MOVEUNALIGNED_LEFT47(dst, src, n, srcofs);
  ^~~~~~~~~~~~~~~~~~~~
rte_memcpy.h:649:51: warning: conversion from 'size_t'
{aka 'long unsigned int'} to 'int' may change value [-Wconversion]
     case 0x0B: MOVEUNALIGNED_LEFT47_IMM(dst, src, n, 0x0B); break;
                                                   ^

rte_memcpy.h:616:15: note: in definition of macro 'MOVEUNALIGNED_LEFT47_IMM'
         tmp = len;
               ^~~
rte_memcpy.h:793:2: note: in expansion of macro 'MOVEUNALIGNED_LEFT47'
  MOVEUNALIGNED_LEFT47(dst, src, n, srcofs);
  ^~~~~~~~~~~~~~~~~~~~
rte_memcpy.h:618:13: warning: conversion to 'size_t'
{aka 'long unsigned int'} from 'int'
may change the sign of the result [-Wsign-conversion]
         tmp -= len;
             ^~

rte_memcpy.h:649:16: note: in expansion of macro 'MOVEUNALIGNED_LEFT47_IMM'
     case 0x0B: MOVEUNALIGNED_LEFT47_IMM(dst, src, n, 0x0B); break;
                ^~~~~~~~~~~~~~~~~~~~~~~~
rte_memcpy.h:793:2: note: in expansion of macro 'MOVEUNALIGNED_LEFT47'
  MOVEUNALIGNED_LEFT47(dst, src, n, srcofs);
  ^~~~~~~~~~~~~~~~~~~~
rte_memcpy.h:618:13: warning: conversion to 'size_t'
{aka 'long unsigned int'} from 'int'
may change the sign of the result [-Wsign-conversion]
             tmp -= len;
                 ^~

We can eliminate the problems by setting the type of tmp to
size_t in the first place.

Fixes: d35cc1fe6a ("eal/x86: revert select optimized memcpy at run-time")
Cc: stable@dpdk.org
Suggested-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: Andy Green <andy@warmcat.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>