net: qrtr: restrict socket creation to the initial network namespace

In the Linux kernel, the following vulnerability has been resolved: net: qrtr: restrict socket creation to the initial network namespace QRTR keeps its entire port and node state in module-global variables that are not partitioned per network namespace: qrtr_local_nid is a single global node id (always 1) and qrtr_ports is a single global xarray. qrtr_port_lookup() and qrtr_local_enqueue() operate on that global state with no network-namespace check, and qrtr_create() places no restriction on the namespace a socket is created in. As a result an unprivileged process that creates an AF_QIPCRTR socket in a separate network namespace, e.g. via unshare(CLONE_NEWUSER | CLONE_NEWNET), can send QRTR datagrams - including control-plane messages such as QRTR_TYPE_NEW_SERVER - to QRTR sockets owned by another namespace, and vice versa. The receiving socket sees such a message as coming from node id 1, indistinguishable from a legitimate local client, breaking the isolation that network namespaces are expected to provide. QRTR is a transport to global hardware endpoints (the modem and other remote processors) and has no per-namespace semantics; its in-kernel name service already creates its socket in init_net only. Confine the socket family to the initial network namespace, as other non-namespace-aware socket families do (see llc_ui_create() and the ieee802154 socket code).