ipc: rename ipcctl_pre_down_nolock()

Both the comment and the name of ipcctl_pre_down_nolock() are misleading:
The function must be called while holdling the rw semaphore.

Therefore the patch renames the function to ipcctl_obtain_check(): This
name matches the other names used in util.c:

- "obtain" function look up a pointer in the idr, without
  acquiring the object lock.
- The caller is responsible for locking.
- _check means that the sequence number is checked.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Manfred Spraul <[email protected]>
Reviewed-by: Davidlohr Bueso <[email protected]>
Cc: Davidlohr Bueso <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Michael Kerrisk <[email protected]>
Cc: Michal Hocko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

ipc/msg.c

@@ -386,7 +386,7 @@ static int msgctl_down(struct ipc_namespace *ns, int msqid, int cmd,
 	down_write(&msg_ids(ns).rwsem);
 	rcu_read_lock();
 
-	ipcp = ipcctl_pre_down_nolock(ns, &msg_ids(ns), msqid, cmd,
+	ipcp = ipcctl_obtain_check(ns, &msg_ids(ns), msqid, cmd,
 				      &msqid64->msg_perm, msqid64->msg_qbytes);
 	if (IS_ERR(ipcp)) {
 		err = PTR_ERR(ipcp);