ipv6 sit: Set relay to 0.0.0.0 directly if relay_prefixlen == 0.

Do not use bit-shift if relay_prefixlen == 0;
relay_prefix << 32 does not result in 0.
Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>