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/shm.c b/ipc/shm.c
index c4fb359..b0eb375 100644
--- a/ipc/shm.c
+++ b/ipc/shm.c
@@ -948,7 +948,7 @@ static int shmctl_ipc_info(struct ipc_namespace *ns,
 		shminfo->shmall = ns->shm_ctlall;
 		shminfo->shmmin = SHMMIN;
 		down_read(&shm_ids(ns).rwsem);
-		err = ipc_get_maxid(&shm_ids(ns));
+		err = ipc_get_maxidx(&shm_ids(ns));
 		up_read(&shm_ids(ns).rwsem);
 		if (err < 0)
 			err = 0;
@@ -968,7 +968,7 @@ static int shmctl_shm_info(struct ipc_namespace *ns,
 		shm_info->shm_tot = ns->shm_tot;
 		shm_info->swap_attempts = 0;
 		shm_info->swap_successes = 0;
-		err = ipc_get_maxid(&shm_ids(ns));
+		err = ipc_get_maxidx(&shm_ids(ns));
 		up_read(&shm_ids(ns).rwsem);
 		if (err < 0)
 			err = 0;