ipc: get rid of ids->tables_initialized hack

ipc: get rid of ids->tables_initialized hack

In sysvipc we have an ids->tables_initialized regarding the rhashtable,
introduced in 0cfb6aee70bd ("ipc: optimize semget/shmget/msgget for lots
of keys")

It's there, specifically, to prevent nil pointer dereferences, from using
an uninitialized api.  Considering how rhashtable_init() can fail
(probably due to ENOMEM, if anything), this made the overall ipc
initialization capable of failure as well.  That alone is ugly, but fine,
however I've spotted a few issues regarding the semantics of
tables_initialized (however unlikely they may be):

- There is inconsistency in what we return to userspace: ipc_addid()
  returns ENOSPC which is certainly _wrong_, while ipc_obtain_object_idr()
  returns EINVAL.

- After we started using rhashtables, ipc_findkey() can return nil upon
  !tables_initialized, but the caller expects nil for when the ipc
  structure isn't found, and can therefore call into ipcget() callbacks.

Now that rhashtable initialization cannot fail, we can properly get rid of
the hack altogether.

[[email protected]: commit id extended to 12 digits]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Davidlohr Bueso <[email protected]>
Signed-off-by: Manfred Spraul <[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]>