netfilter: nft_flow_offload: Fix reverse route lookup

Using the following example:

	client 1.1.1.7 ---> 2.2.2.7 which dnat to 10.0.0.7 server

The first reply packet (ie. syn+ack) uses an incorrect destination
address for the reverse route lookup since it uses:

	daddr = ct->tuplehash[!dir].tuple.dst.u3.ip;

which is 2.2.2.7 in the scenario that is described above, while this
should be:

	daddr = ct->tuplehash[dir].tuple.src.u3.ip;

that is 10.0.0.7.

Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>