ipcns: fix use after free in free_ipc_ns()

commit b515498 ("userns: add a user namespace owner of ipc ns") added a
user namespace owner of ipc ns, but it also introduced a use after free in
free_ipc_ns().

Signed-off-by: Xiaotian Feng <[e-mail]>
Acked-by: "Serge E. Hallyn" <[e-mail]>
Acked-by: David Howells <[e-mail]>
Cc: "Eric W. Biederman" <[e-mail]>
Cc: Daniel Lezcano <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

ipc/namespace.c

···
 	sem_exit_ns(ns);
 	msg_exit_ns(ns);
 	shm_exit_ns(ns);
-	kfree(ns);
 	atomic_dec(&nr_ipc_ns);
 
 	/*
···
 	 */
 	ipcns_notify(IPCNS_REMOVED);
 	put_user_ns(ns->user_ns);
+	kfree(ns);
 }
 
 /*