netfilter: nf_tables: don't use 'data_size' uninitialized

Commit 2c865a8a28 ("netfilter: nf_tables: add rule blob layout") never
initialized the new 'data_size' variable.

I'm not sure how it ever worked, but it might have worked almost by
accident - gcc seems to occasionally miss these kinds of 'variable used
uninitialized' situations, but I've seen it do so because it ended up
zero-initializing them due to some other simplification.

But clang is very unhappy about it all, and correctly reports

    net/netfilter/nf_tables_api.c:8278:4: error: variable 'data_size' is uninitialized when used here [-Werror,-Wuninitialized]
                            data_size += sizeof(*prule) + rule->dlen;
                            ^~~~~~~~~
    net/netfilter/nf_tables_api.c:8263:30: note: initialize the variable 'data_size' to silence this warning
            unsigned int size, data_size;
                                        ^
                                         = 0
    1 error generated.

and this fix just initializes 'data_size' to zero before the loop.

Fixes: 2c865a8a28 ("netfilter: nf_tables: add rule blob layout")
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: David Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

net/netfilter/nf_tables_api.c

@@ -8273,6 +8273,7 @@ static int nf_tables_commit_chain_prepare(struct net *net, struct nft_chain *cha
 	rule = list_entry(&chain->rules, struct nft_rule, list);
 	i = 0;
 
+	data_size = 0;
 	list_for_each_entry_continue(rule, &chain->rules, list) {
 		if (nft_is_active_next(net, rule)) {
 			data_size += sizeof(*prule) + rule->dlen;