netfilter: nf_tables: remove assignment with no effect in chain blob builder

cppcheck possible warnings:

>> net/netfilter/nf_tables_api.c:2014:2: warning: Assignment of function parameter has no effect outside the function. Did you forget dereferencing it? [uselessAssignmentPtrArg]
    ptr += offsetof(struct nft_rule_dp, data);
    ^

Reported-by: kernel test robot <yujie.liu@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>