netfilter: nf_log: Introduce nft_log_dereference() macro

[ Upstream commit 0c26ed1c07f13ca27e2638ffdd1951013ed96c48 ]

Wrap up a common call pattern in an easier to handle call.

Signed-off-by: Marcelo Ricardo Leitner <mleitner@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>