netfilter: nft_set_hash: add nft_hash_buckets()

Add nft_hash_buckets() helper function to calculate the number of
hashtable buckets based on the elements. This function can be reused
from the follow up patch to add non-resizable hashtables.
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_set_hash.c

@@ -363,12 +363,17 @@ static void nft_rhash_destroy(const struct nft_set *set)
 				    (void *)set);
 }
 
+static u32 nft_hash_buckets(u32 size)
+{
+	return roundup_pow_of_two(size * 4 / 3);
+}
+
 static bool nft_rhash_estimate(const struct nft_set_desc *desc, u32 features,
 			       struct nft_set_estimate *est)
 {
 	if (desc->size)
 		est->size = sizeof(struct nft_rhash) +
-			    roundup_pow_of_two(desc->size * 4 / 3) *
+			    nft_hash_buckets(desc->size) *
 			    sizeof(struct nft_rhash_elem *) +
 			    desc->size * sizeof(struct nft_rhash_elem);
 	else