UPSTREAM: cgroup: Elide write-locking threadgroup_rwsem when updating csses on an empty subtree

cgroup_update_dfl_csses() write-lock the threadgroup_rwsem as updating the
csses can trigger process migrations. However, if the subtree doesn't
contain any tasks, there aren't gonna be any cgroup migrations. This
condition can be trivially detected by testing whether
mgctx.preloaded_src_csets is empty. Elide write-locking threadgroup_rwsem if
the subtree is empty.

After this optimization, the usage pattern of creating a cgroup, enabling
the necessary controllers, and then seeding it with CLONE_INTO_CGROUP and
then removing the cgroup after it becomes empty doesn't need to write-lock
threadgroup_rwsem at all.

Bug: 242685775
Change-Id: Ifc96030fc7b0655ecd85ef19c52c9ed97e910ffb
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
(cherry picked from commit 671c11f0
 https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git

 master)
Signed-off-by: Mukesh Ojha <quic_mojha@quicinc.com>
(cherry picked from commit a30c0961)