[NETFILTER]: hashlimit match: fix random initialization

hashlimit does:

        if (!ht->rnd)
                get_random_bytes(&ht->rnd, 4);

ignoring that 0 is also a valid random number.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>