cgroup: drop the matching uid requirement on migration for cgroup v2

Along with the write access to the cgroup.procs or tasks file, cgroup
has required the writer's euid, unless root, to match [s]uid of the
target process or task.  On cgroup v1, this is necessary because
there's nothing preventing a delegatee from pulling in tasks or
processes from all over the system.

If a user has a cgroup subdirectory delegated to it, the user would
have write access to the cgroup.procs or tasks file.  If there are no
further checks than file write access check, the user would be able to
pull processes from all over the system into its subhierarchy which is
clearly not the intended behavior.  The matching [s]uid requirement
partially prevents this problem by allowing a delegatee to pull in the
processes that belongs to it.  This isn't a sufficient protection
however, because a user would still be able to jump processes across
two disjoint sub-hierarchies that has been delegated to them.

cgroup v2 resolves the issue by requiring the writer to have access to
the common ancestor of the cgroup.procs file of the source and target
cgroups.  This confines each delegatee to their own sub-hierarchy
proper and bases all permission decisions on the cgroup filesystem
rather than having to pull in explicit uid matching.

cgroup v2 has still been applying the matching [s]uid requirement just
for historical reasons.  On cgroup2, the requirement doesn't serve any
purpose while unnecessarily complicating the permission model.  Let's
drop it.

Signed-off-by: Tejun Heo <tj@kernel.org>