netfilter: nf_tables: remove unused variable

> Remove unused variable and fix missing initialization.
>
> >> net/netfilter/nf_tables_api.c:8266:6: warning: variable 'i' set but not used [-Wunused-but-set-variable]
>            int i;
>                ^

Fixes: 2c865a8a ("netfilter: nf_tables: add rule blob layout")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>