writeback, cgroup: remove extra percpu_ref_exit()

5.10 stable commit 2b00b2a0 ("writeback, cgroup: fix null-ptr-deref
write in bdi_split_work_to_wbs") is a backport of upstream 6.3 commit
1ba1199e.

In the 5.10 stable commit backport percpu_ref_exit() is called twice:
first in cgwb_release_workfn() and then in cgwb_free_rcu(). The 2nd call
is benign as percpu_ref_exit() internally detects there's nothing to do.

This fixes an non-upstream issue that only applies to 5.10.y.

Fixes: 2b00b2a0 ("writeback, cgroup: fix null-ptr-deref write in bdi_split_work_to_wbs")
Signed-off-by: Greg Thelen <gthelen@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>