psi: link legacy root to psi_system

Upstream: no, unless upstream decide to support cgroup v1 PSI

* This is part of EMM *

When interating through cgroup for PSI, the cgroup->psi must be
non-NULL. For rootcg case, v2 can set the static cgrp_dfl_root's psi to
point to psi_system. v1 have to set the pointer to psi_system for each
mounted system.

Signed-off-by: Kairui Song <kasong@tencent.com>

kernel/cgroup/cgroup.c

@@ -2079,6 +2079,7 @@ int cgroup_setup_root(struct cgroup_root *root, u16 ss_mask)
 	root_cgrp->kn = kernfs_root_to_node(root->kf_root);
 	WARN_ON_ONCE(cgroup_ino(root_cgrp) != 1);
 	root_cgrp->ancestors[0] = root_cgrp;
+	root_cgrp->psi = &psi_system;
 
 	ret = css_populate_dir(&root_cgrp->self);
 	if (ret)