pktgen: do not abuse IN6_ADDR_HSIZE

pktgen accidentally used IN6_ADDR_HSIZE, instead of using the size of an
IPv6 address.

Since IN6_ADDR_HSIZE recently was increased from 16 to 256, this old
bug is hitting us.

Fixes: 3f27fb23 ("ipv6: addrconf: add per netns perturbation in inet6_addr_hash()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>