netfilter: nft_flow_offload: reset dst in route object after setting up flow

In the Linux kernel, the following vulnerability has been resolved: netfilter: nft_flow_offload: reset dst in route object after setting up flow dst is transferred to the flow object, route object does not own it anymore. Reset dst in route object, otherwise if flow_offload_add() fails, error path releases dst twice, leading to a refcount underflow.