cpuset: initialize top_cpuset's configured masks at mount

We now have to support different behaviors for default hierachy and
legacy hiearchy, top_cpuset's configured masks need to be initialized
accordingly.

Suppose we've offlined cpu1.

On default hierarchy:

	# mount -t cgroup -o __DEVEL__sane_behavior xxx /cpuset
	# cat /cpuset/cpuset.cpus
	0-15

On legacy hierarchy:

	# mount -t cgroup xxx /cpuset
	# cat /cpuset/cpuset.cpus
	0,2-15

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>