[stable] locks: fix a potential use-after-free problem when wakeup a waiter

This commit (included in 5.6-rc5) seems to be needed for 5.4 and 5.5 branches:

commit 6d390e4b5d48ec03bb87e63cf0a2bff5f4e116da Author: yangerkun yangerkun@huawei.com Date: Wed Mar 4 15:25:56 2020 +0800

locks: fix a potential use-after-free problem when wakeup a waiter

I'm a bit surprised that it hasn't yet been applied, while some fixes from 5.6-rc6 have.

Ben.