cgroup: fix seq_show_option merge with legacy_name

When seq_show_option (commit a068acf2: "fs: create and use
seq_show_option for escaping") was merged, it did not correctly collide
with cgroup's addition of legacy_name (commit 3e1d2eed: "cgroup:
introduce cgroup_subsys->legacy_name") changes.

This fixes the reported name.
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/cgroup.c

@@ -1342,7 +1342,7 @@ static int cgroup_show_options(struct seq_file *seq,
 	if (root != &cgrp_dfl_root)
 		for_each_subsys(ss, ssid)
 			if (root->subsys_mask & (1 << ssid))
-				seq_show_option(seq, ss->name, NULL);
+				seq_show_option(seq, ss->legacy_name, NULL);
 	if (root->flags & CGRP_ROOT_NOPREFIX)
 		seq_puts(seq, ",noprefix");
 	if (root->flags & CGRP_ROOT_XATTR)