netfilter: nf_tables: initialize registers in nft_do_chain()

commit 4c905f67 upstream.

Initialize registers to avoid stack leak into userspace.

Fixes: 96518518 ("netfilter: add nftables")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>