net: fix clang build

net: fix clang build

Interestingly, clang and gcc has different prototype for _mm_prefetch().
For gcc, we have

   _mm_prefetch (const void *__P, enum _mm_hint __I)

While for clang, it's

   #define _mm_prefetch(a, sel) (__builtin_prefetch((void *)(a), 0, (sel)))

That's how the following error comes with clang:

   error: cast from 'const void *' to 'void *' drops const qualifier
   [-Werror,-Wcast-qual]
           _mm_prefetch((const void *)rused, _MM_HINT_T0);
   /usr/lib/llvm-3.8/bin/../lib/clang/3.8.0/include/xmmintrin.h:684:58:
   note: expanded from macro '_mm_prefetch'
            #define _mm_prefetch(a, sel) (__builtin_prefetch((void *)(a),
                                          0, (sel)))

What's weird is that the build was actaully Okay before. I met it while
apply Jerin's vector support for ARM patch set: he just move this piece
of code to another file, nothing else changed.

This patch fix the issue when Jerin's patchset is applied. Thus, I think
it's still needed.

Similarly, make the same change to other _mm_prefetch users, just in case
this weird issue shows up again somehow later.

Fixes: fc3d66212fed ("virtio: add vector Rx")
Fixes: c95584dc2b18 ("ixgbe: new vectorized functions for Rx/Tx")
Fixes: 9ed94e5bb04e ("i40e: add vector Rx")
Fixes: 7092be8437bd ("fm10k: add vector Rx")
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Jing Chen <jing.d.chen@intel.com>