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

Commit 2c865a8a28a1 ("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: 2c865a8a28a1 ("netfilter: nf_tables: add rule blob layout")
Cc: Pablo Neira Ayuso <[e-mail]>
Cc: Jakub Kicinski <[e-mail]>
Cc: David Miller <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

net/netfilter/nf_tables_api.c

···
 	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;