selftests: cgroup: Test open-time cgroup namespace usage for migration checks

selftests: cgroup: Test open-time cgroup namespace usage for migration checks

BugLink: https://bugs.launchpad.net/bugs/1969107
commit bf35a7879f1dfb0d050fe779168bcf25c7de66f5 upstream.

When a task is writing to an fd opened by a different task, the perm check
should use the cgroup namespace of the latter task. Add a test for it.

Tested-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 4693fce5a5d461ed6cdcce12ec37bbf5cabab699)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>