proc: test /proc/sysvipc vs setns(CLONE_NEWIPC)

I thought that /proc/sysvipc has the same bug as /proc/net

commit 1fde6f21d90f8ba5da3cb9c54ca991ed72696c43
proc: fix /proc/net/* after setns(2)

However, it doesn't! /proc/sysvipc files do

get_ipc_ns(current->nsproxy->ipc_ns);

in their open() hook and avoid the problem.

Keep the test, maybe /proc/sysvipc will become broken someday :-\

Link: http://lkml.kernel.org/r/20190706180146.GA21015@avx2
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>