ipc/util.h: update documentation for ipc_getref() and ipc_putref()

Now that ipc_rcu_alloc() and ipc_rcu_free() are removed,
document when it is valid to use ipc_getref() and ipc_putref().

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Manfred Spraul <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Kees Cook <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

ipc/util.h

@@ -112,6 +112,9 @@ int ipcperms(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp, short flg);
  * Objects are reference counted, they start with reference count 1.
  * getref increases the refcount, the putref call that reduces the recount
  * to 0 schedules the rcu destruction. Caller must guarantee locking.
+ *
+ * refcount is initialized by ipc_addid(), before that point call_rcu()
+ * must be used.
  */
 int ipc_rcu_getref(struct kern_ipc_perm *ptr);
 void ipc_rcu_putref(struct kern_ipc_perm *ptr,