Revert "cgroup: enable cgroup_get_from_file() on cgroup1"

This reverts commit f3a2aeb.

The commit enabled looking up v1 cgroups via cgroup_get_from_file().
However, there are multiple users, including CLONE_INTO_CGROUP, which have
been assuming that it would only look up v2 cgroups. Returning v1 cgroups
breaks them.

Let's revert the commit and retry later with a separate lookup interface
which allows both v1 and v2.

Signed-off-by: Tejun Heo <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Cc: Yosry Ahmed <[email protected]>

Author: htejun

kernel/cgroup/cgroup.c

@@ -6234,6 +6234,11 @@ static struct cgroup *cgroup_get_from_file(struct file *f)
 		return ERR_CAST(css);
 
 	cgrp = css->cgroup;
+	if (!cgroup_on_dfl(cgrp)) {
+		cgroup_put(cgrp);
+		return ERR_PTR(-EBADF);
+	}
+
 	return cgrp;
 }