ns_tree_lookup_rcu
ns = ns_tree_lookup_rcu(mnt_ns_id, CLONE_NEWNS);
ns = ns_tree_lookup_rcu(fid->ns_id, fid->ns_type);
struct ns_common *ns_tree_lookup_rcu(u64 ns_id, int ns_type);
ns = ns_tree_lookup_rcu(mnt_ns_id, ns_type);