cgroup: release cgroup_mutex over file removals

Now that cftypes and all tree modification operations are protected by
cgroup_tree_mutex, we can drop cgroup_mutex while deleting files and
directories.  Drop cgroup_mutex over removals.

This doesn't make any noticeable difference now but is to help kernfs
conversion.  In kernfs, removals are sync points which drain in-flight
operations as those operations would grab cgroup_mutex, trying to
delete under cgroup_mutex would deadlock.  This can be resolved by
just holding the outer cgroup_tree_mutex which nests outside both
kernfs active reference and cgroup_mutex.

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