ipc/shm: properly return EIDRM in shm_lock()

When getting rid of the general ipc_lock(), this was missed furthermore,
making the comment around the ipc object validity check bogus.  Under
EIDRM conditions, callers will in turn not see the error and continue
with the operation.

Link: http://lkml.kernel.org/r/20180824030920.GD3677@linux-r8p5
Link: http://lkml.kernel.org/r/20180823024051.GC13343@shao2-debian
Fixes: 82061c5 ("ipc: drop ipc_lock()")
Signed-off-by: Davidlohr Bueso <[email protected]>
Reported-by: kernel test robot <[email protected]>
Cc: Manfred Spraul <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Davidlohr Bueso

ipc/shm.c

@@ -199,6 +199,7 @@ static inline struct shmid_kernel *shm_lock(struct ipc_namespace *ns, int id)
 	}
 
 	ipc_unlock_object(ipcp);
+	ipcp = ERR_PTR(-EIDRM);
 err:
 	rcu_read_unlock();
 	/*