lib/libnet/ipv6: Silence compiler warning from Clang

When compiling the libnet code with Clang (e.g. for the s390-ccw bios),
it complains with the following warning:

 ipv6.c:447:18: warning: variable length array folded to constant array
  as an extension [-Wgnu-folding-constant]
                unsigned short raw[ip6size];
                               ^
The warning is completely harmless, of course. Anyway let's rewrite the
code a little bit to make the compiler silent again.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>