cgroup: move number_of_cgroups test out of rebind_subsystems() into cgroup_remount()

rebind_subsystems() currently fails if the hierarchy has any !root
cgroups; however, on the planned unified hierarchy,
rebind_subsystems() will be used while populated.  Move the test to
cgroup_remount(), which is the only place the test is necessary
anyway.

As it's impossible for the other two callers of rebind_subsystems() to
have populated hierarchy, this doesn't make any behavior changes.

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