netfilter: nf_tables: disallow element updates of bound anonymous sets

Anonymous sets come with NFT_SET_CONSTANT from userspace. Although API
allows to create anonymous sets without NFT_SET_CONSTANT, it makes no
sense to allow to add and to delete elements for bound anonymous sets.

Fixes: 96518518 ("netfilter: add nftables")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>