netfilter: nft_dynset: fix panic if NFT_SET_HASH is not enabled

When CONFIG_NFT_SET_HASH is not enabled and I input the following rule:
"nft add rule filter output flow table test {ip daddr counter }", kernel
panic happened on my system:
 BUG: unable to handle kernel NULL pointer dereference at (null)
 IP: [<          (null)>]           (null)
 [...]
 Call Trace:
 [<ffffffffa0590466>] ? nft_dynset_eval+0x56/0x100 [nf_tables]
 [<ffffffffa05851bb>] nft_do_chain+0xfb/0x4e0 [nf_tables]
 [<ffffffffa0432f01>] ? nf_conntrack_tuple_taken+0x61/0x210 [nf_conntrack]
 [<ffffffffa0459ea6>] ? get_unique_tuple+0x136/0x560 [nf_nat]
 [<ffffffffa043bca1>] ? __nf_ct_ext_add_length+0x111/0x130 [nf_conntrack]
 [<ffffffffa045a357>] ? nf_nat_setup_info+0x87/0x3b0 [nf_nat]
 [<ffffffff81761e27>] ? ipt_do_table+0x327/0x610
 [<ffffffffa045a6d7>] ? __nf_nat_alloc_null_binding+0x57/0x80 [nf_nat]
 [<ffffffffa059f21f>] nft_ipv4_output+0xaf/0xd0 [nf_tables_ipv4]
 [<ffffffff81702515>] nf_iterate+0x55/0x60
 [<ffffffff81702593>] nf_hook_slow+0x73/0xd0

Because in rbtree type set, ops->update is not implemented. So just keep
it simple, in such case, report -EOPNOTSUPP to the user space.

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

net/netfilter/nft_dynset.c

@@ -139,6 +139,9 @@ static int nft_dynset_init(const struct nft_ctx *ctx,
 			return PTR_ERR(set);
 	}
 
+	if (set->ops->update == NULL)
+		return -EOPNOTSUPP;
+
 	if (set->flags & NFT_SET_CONSTANT)
 		return -EBUSY;