device_cgroup: don't grab mutex in rcu callback

commit 103a197 "security/device_cgroup: lock assert fails in
dev_exception_clean()" grabs devcgroup_mutex to fix assert failure,
but mutex can't be grabbed in rcu callback. Since there shouldn't be
any other references when css_free is called, mutex isn't needed for
list cleanup in devcgroup_css_free().

Signed-off-by: Jerry Snitselaar <jerry.snitselaar@oracle.com>