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>