netfilter: nft_dynset: report EOPNOTSUPP on missing set feature

If userspace requests a feature which is not available the original set
definition, then bail out with EOPNOTSUPP. If userspace sends
unsupported dynset flags (new feature not supported by this kernel),
then report EOPNOTSUPP to userspace. EINVAL should be only used to
report malformed netlink messages from userspace.

Fixes: 22fe54d5 ("netfilter: nf_tables: add support for dynamic set updates")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>