ipc/util.c: further variable name cleanups

The varable names got a mess, thus standardize them again:

id: user space id. Called semid, shmid, msgid if the type is known.
    Most functions use "id" already.
idx: "index" for the idr lookup
    Right now, some functions use lid, ipc_addid() already uses idx as
    the variable name.
seq: sequence number, to avoid quick collisions of the user space id
key: user space key, used for the rhash tree

Link: http://lkml.kernel.org/r/20180712185241.4017-12-manfred@colorfullife.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Davidlohr Bueso <dbueso@suse.de>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Kees Cook <keescook@chromium.org>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/ipc/msg.c b/ipc/msg.c
index ee78dad..883642c 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -456,7 +456,7 @@ static int msgctl_info(struct ipc_namespace *ns, int msqid,
 			 int cmd, struct msginfo *msginfo)
 {
 	int err;
-	int max_id;
+	int max_idx;
 
 	/*
 	 * We must not return kernel stack data.
@@ -483,9 +483,9 @@ static int msgctl_info(struct ipc_namespace *ns, int msqid,
 		msginfo->msgpool = MSGPOOL;
 		msginfo->msgtql = MSGTQL;
 	}
-	max_id = ipc_get_maxid(&msg_ids(ns));
+	max_idx = ipc_get_maxidx(&msg_ids(ns));
 	up_read(&msg_ids(ns).rwsem);
-	return (max_id < 0) ? 0 : max_id;
+	return (max_idx < 0) ? 0 : max_idx;
 }
 
 static int msgctl_stat(struct ipc_namespace *ns, int msqid,