netfilter: nf_tables: add nft_dup expression

netfilter: nf_tables: add nft_dup expression

This new expression uses the nf_dup engine to clone packets to a given gateway.
Unlike xt_TEE, we use an index to indicate output interface which should be
fine at this stage.

Moreover, change to the preemtion-safe this_cpu_read(nf_skb_duplicated) from
nf_dup_ipv{4,6} to silence a lockdep splat.

Based on the original tee expression from Arturo Borrero Gonzalez, although
this patch has diverted quite a bit from this initial effort due to the
change to support maps.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
(cherry picked from commit d877f07112f1e5a247c6b585c971a93895c9f738)

Orabug: 24694570

Signed-off-by: Ethan Zhao <ethan.zhao@oracle.com>
Reviewed-by: Dhaval Giani <dhaval.giani@oracle.com>