Move the user namespace at the first position in the array so that we always
attach to it first when iterating over the struct and using setns() to switch
namespaces. This especially affects lxc_attach(): Suppose you cloned a new user
namespace and mount namespace as an unprivileged user on the host and want to
setns() to the mount namespace. This requires you to attach to the user
namespace first otherwise the kernel will fail this check:

    if (!ns_capable(mnt_ns->user_ns, CAP_SYS_ADMIN) ||
        !ns_capable(current_user_ns(), CAP_SYS_CHROOT) ||
        !ns_capable(current_user_ns(), CAP_SYS_ADMIN))
    	return -EPERM;

in

    linux/fs/namespace.c:mntns_install().
Signed-off-by: Christian Brauner <christian.brauner@canonical.com>