rhashtable: provide len to obj_hashfn

nftables sets will be converted to use so called setextensions, moving
the key to a non-fixed position. To hash it, the obj_hashfn must be used,
however it so far doesn't receive the length parameter.

Pass the key length to obj_hashfn() and convert existing users.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

lib/rhashtable.c

@@ -691,7 +691,7 @@ static u32 rhashtable_jhash2(const void *key, u32 length, u32 seed)
  *	struct rhash_head	node;
  * };
  *
- * u32 my_hash_fn(const void *data, u32 seed)
+ * u32 my_hash_fn(const void *data, u32 len, u32 seed)
  * {
  *	struct test_obj *obj = data;
  *