livepatch: Optimize list_del operation in 'arch_klp_unpatch_func'

hulk inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I53WZ9



--------------------------------

Currently when unpatch a function, we check whether 'func_stack' has only
one item then delete it:
  > if (list_is_singular(&func_node->func_stack)) {
  > 	list_del_rcu(&func->stack_node);
  > 	......
  > } else {
  > 	list_del_rcu(&func->stack_node);
  > 	next_func = list_first_or_null_rcu(&func_node->func_stack);
  > 	......
  > }

We can optimize it as delete first then check whether 'func_stack' is
empty or not.

Suggested-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Zheng Yejian <zhengyejian1@huawei.com>
Reviewed-by: Kuohai Xu <xukuohai@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>