netfilter: nf_tables: Carry s_idx in nft_obj_dump_ctx

[ Upstream commit 2eda95cf ]

Prep work for moving the context into struct netlink_callback scratch
area.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: bd662c42 ("netfilter: nf_tables: Add locking for NFT_MSG_GETOBJ_RESET requests")
Signed-off-by: Sasha Levin <sashal@kernel.org>