ipc/msg.c: avoid ipc_rcu_putref for failed ipc_addid()

ipc/msg.c: avoid ipc_rcu_putref for failed ipc_addid()

Loosely based on a patch from Kees Cook <keescook@chromium.org>:
 - id and retval can be merged
 - if ipc_addid() fails, then use call_rcu() directly.

The difference is that call_rcu is used for failed ipc_addid() calls, to
continue to guaranteed an rcu delay for security_msg_queue_free().

Link: http://lkml.kernel.org/r/20170525185107.12869-16-manfred@colorfullife.com
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>