netfilter: nf_tables: Introduce nf_tables_getrule_single()

[ Upstream commit 1578c328 ]

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

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Stable-dep-of: 9adbb419 ("netfilter: nf_tables: avoid false-positive lockdep splat on rule deletion")
Signed-off-by: Sasha Levin <sashal@kernel.org>