Disallow more mount options if sane_behavior.  Note that xattr used to
generate warning.

While at it, simplify option check in cgroup_mount() and update
sane_behavior comment in cgroup.h.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>