flow_offload_alloc() calls nf_route() to get a dst_entry. Internally,
nf_route() calls ip_route_output_key() that allocates a dst_entry and
holds it. So, a dst_entry should be released by dst_release() if
nf_route() is successful.
Otherwise, netns exit routine cannot be finished and the following
message is printed:
[ 257.490952] unregister_netdevice: waiting for lo to become free. Usage count = 1