netfilter: nf_tables: don't store address of last rule on jump

Walk the rule headers until the trailer one (last_bit flag set) instead
of stopping at last_rule address.

This avoids the need to store the address when jumping to another chain.

This cuts size of jumpstack array by one third, on 64bit from
384 to 256 bytes.  Still, stack usage is still quite large:

scripts/stackusage:
nf_tables_core.c:258 nft_do_chain    496     static

Next patch will also remove chain pointer.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>