psp: fix NULL genl_sock deref race with concurrent netns teardown

psp: fix NULL genl_sock deref race with concurrent netns teardown

The race occurs between network namespace removal and PSP device
unregistration.  When a netns is deleted while a PSP device associated
with that netns is concurrently being removed, psp_dev_unregister()
triggers psp_nl_notify_dev() to send a device change notification.
Concurrently, cleanup_net() running in the netns workqueue calls
genl_pernet_exit(), which sets net->genl_sock to NULL. If
genl_pernet_exit() wins the race, two sites in psp_nl_multicast_per_ns()
then dereference the NULL socket and crash:

CPU 0 (netns teardown)       CPU 1 (PSP device unregister)
======================       =============================
cleanup_net [workqueue]
  genl_pernet_exit()         psp_dev_unregister()
    net->genl_sock = NULL      psp_nl_notify_dev()
                                 psp_nl_multicast_per_ns()
                                   build_ntf()
                                     -> netlink_has_listeners(NULL)
                                     /* crash */
                                   genlmsg_multicast_netns()
                                     -> nlmsg_multicast_filtered(NULL)
                                     /* crash */

Fix by replacing the bare dev_net() calls with maybe_get_net().
maybe_get_net() returns NULL if the namespace is already dying.
Holding the reference ensures genl_sock remains valid across both the
build_ntf() and genlmsg_multicast_netns() calls.

Fixes: 00c94ca2b99e ("psp: base PSP device support")
Fixes: 06c2dce2d0f6 ("psp: add new netlink cmd for dev-assoc and dev-disassoc")
Reviewed-by: Ajit Khaparde <ajit.khaparde@broadcom.com>
Reviewed-by: Vikas Gupta <vikas.gupta@broadcom.com>
Reviewed-by: Bhargava Marreddy <bhargava.marreddy@broadcom.com>
Reviewed-by: Akhilesh Samineni <akhilesh.samineni@broadcom.com>
Signed-off-by: Kiran Kella <kiran.kella@broadcom.com>
Link: https://patch.msgid.link/20260727101628.502042-1-kiran.kella@broadcom.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>