netfilter: nft_dynset: honor stateful expressions in set definition

commit fca05d4d upstream.

If the set definition contains stateful expressions, allocate them for
the newly added entries from the packet path.

[ This backport includes nft_set_elem_expr_clone() which has been
  taken from 8cfd9b0f ("netfilter: nftables: generalize set
  expressions support") and skip redundant expressions when set
  already provides it per ce537996 ("netfilter: nft_dynset: dump
  expressions when set definition contains no expressions") ]

Fixes: 65038428 ("netfilter: nf_tables: allow to specify stateful expression in set definition")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>