netfilter: nf_tables: Introduce nf_tables_getobj_single

[ Upstream commit 69fc3e9e ]

Outsource the reply skb preparation for non-dump getrule requests into a
distinct function. Prep work for object reset locking.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Reviewed-by: Florian Westphal <fw@strlen.de>
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>