[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: NEric Dumazet <dada1@cosmosbay.com>
Signed-off-by: NPatrick McHardy <kaber@trash.net>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/netfilter/xt_hashlimit.c

@@ -105,7 +105,16 @@ static inline bool dst_cmp(const struct dsthash_ent *ent,
 static u_int32_t
 hash_dst(const struct xt_hashlimit_htable *ht, const struct dsthash_dst *dst)
 {
-	return jhash(dst, sizeof(*dst), ht->rnd) % ht->cfg.size;
+	u_int32_t hash = jhash2((const u32 *)dst,
+				sizeof(*dst)/sizeof(u32),
+				ht->rnd);
+	/*
+	 * Instead of returning hash % ht->cfg.size (implying a divide)
+	 * we return the high 32 bits of the (hash * ht->cfg.size) that will
+	 * give results between [0 and cfg.size-1] and same hash distribution,
+	 * but using a multiply, less expensive than a divide
+	 */
+	return ((u64)hash * ht->cfg.size) >> 32;
 }
 
 static struct dsthash_ent *