net/virtio: fix 32-bit build with gcc 6

net/virtio: fix 32-bit build with gcc 6

This is for target i686-native-linuxapp-gcc and gcc6,

Compilation error is:

In file included from
  include/rte_mempool.h:77:0, from
  drivers/net/virtio/virtio_rxtx_simple.c:
In function `virtio_xmit_pkts_simple':
  include/rte_memcpy.h:551:2: error:
    array subscript is above array bounds
      rte_mov16((uint8_t *)dst + 1 * 16, (const uint8_t *)src + 1 * 16);
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Call stack is as following:

virtio_xmit_pkts_simple
  virtio_xmit_cleanup
    rte_mempool_put_bulk
      rte_mempool_generic_put
        __mempool_generic_put
	  rte_memcpy

The array used as source buffer in virtio_xmit_cleanup (free) is a
pointer array with 32 elements, in 32bit this makes 128 bytes.

in rte_memcpy() implementation, there a code piece as following:
if (size > 256) {
    rte_move128(...);
    rte_move128(...); <--- [1]
    ....
}

The compiler traces the array all through the call stack and knows the
size of array is 128 and generates a warning on above [1] which tries to
access beyond byte 128.
But unfortunately it ignores the "(size > 256)" check.

Giving a hint to compiler that variable "size" is related to the size of
the source buffer fixes compiler warning.

Fixes: 863bfb474493 ("mempool: optimize copy in cache")

Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>
Acked-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>