netlink_linearize: skip set element expression in set statement key

netlink_linearize: skip set element expression in set statement key

Before this patch the following fails:

# nft add rule ip6 filter x \
	set add ip6 saddr . ip6 daddr @test
nft: netlink_linearize.c:648: netlink_gen_expr: Assertion `dreg < ctx->reg_low' failed.
Aborted

This is was previously fixed for flow statements in fbea4a6f4449
("netlink_linearize: skip set element expression in flow table key"), and
this patch implements the same change for set statements by using the set
element key in netlink_gen_set_stmt().

nft-test.py is updated to support set types with concatenated data types
in order to support testing of this.

Signed-off-by: Anders K. Pedersen <akp@cohaesio.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>