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

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

BugLink: https://bugs.launchpad.net/bugs/1969107
commit 613e040e4dc285367bff0f8f75ea59839bc10947 upstream.

When a task is writing to an fd opened by a different task, the perm check
should use the credentials 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 47802775bc119658e59199d859ba31d62dc5e826)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>