Don't allow access to the kernel keyring

For now, we don't use the kernel keyring for anything, so it is
better to shut off the access to it.

common/flatpak-run.c

@@ -3026,6 +3026,11 @@ setup_seccomp (GPtrArray  *argv_array,
     /* Don't allow reading current quota use */
     {SCMP_SYS (quotactl)},
 
+    /* Don't allow access to the kernel keyring */
+    {SCMP_SYS (add_key)},
+    {SCMP_SYS (keyctl)},
+    {SCMP_SYS (request_key)},
+
     /* Scary VM/NUMA ops */
     {SCMP_SYS (move_pages)},
     {SCMP_SYS (mbind)},