[NETFILTER]: xt_hashlimit: speedup hash_dst()

1) Using jhash2() instead of jhash() is a litle bit faster if applicable.

2) Thanks to jhash, hash value uses full 32 bits.
   Instead of returning hash % size (implying a divide)
   we return the high 32 bits of the (hash * size) that will
   give results between [0 and size-1] and same hash distribution.

  On most cpus, a multiply is less expensive than a divide, by an order
  of magnitude.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>