cgroup: update locking in cgroup_show_options()

cgroup_show_options() grabs cgroup_root_mutex to protect the options
changing while printing; however, holding root_mutex or not doesn't
really make much difference for the function.  subsys_mask can be
atomically tested and most of the options aren't allowed to change
anyway once mounted.

The only field which needs synchronization is ->release_agent_path.
This patch introduces a dedicated spinlock to synchronize accesses to
the field and drops cgroup_root_mutex locking from
cgroup_show_options().  The next patch will remove cgroup_root_mutex.

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