ipc: fix ipc data structures inconsistency

As described in the title, this patch fixes <ipc>id_ds inconsistency when
<ipc>ctl_stat executes concurrently with some ds-changing function, e.g.
shmat, msgsnd or whatever.

For instance, if shmctl(IPC_STAT) is running concurrently
with shmat, following data structure can be returned:
{... shm_lpid = 0, shm_nattch = 1, ...}

Link: http://lkml.kernel.org/r/20171202153456.6514-1-philippe.mikoyan@skat.systems
Signed-off-by: Philippe Mikoyan <philippe.mikoyan@skat.systems>
Reviewed-by: Davidlohr Bueso <dbueso@suse.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Manfred Spraul <manfred@colorfullife.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

ipc/util.c

@@ -23,9 +23,12 @@
  *	    tree.
  *	    - perform initial checks (capabilities, auditing and permission,
  *	      etc).
- *	    - perform read-only operations, such as STAT, INFO commands.
+ *	    - perform read-only operations, such as INFO command, that
+ *	      do not demand atomicity
  *	      acquire the ipc lock (kern_ipc_perm.lock) through
  *	      ipc_lock_object()
+ *		- perform read-only operations that demand atomicity,
+ *		  such as STAT command.
  *		- perform data updates, such as SET, RMID commands and
  *		  mechanism-specific operations (semop/semtimedop,
  *		  msgsnd/msgrcv, shmat/shmdt).