srcu: Apply *_ONCE() to ->srcu_last_gp_end

mainline inclusion
from mainline-v5.6-rc1
commit 844a378d
category: bugfix
bugzilla: 28851
CVE: NA

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The ->srcu_last_gp_end field is accessed from any CPU at any time
by synchronize_srcu(), so non-initialization references need to use
READ_ONCE() and WRITE_ONCE().  This commit therefore makes that change.

Reported-by:  <syzbot+08f3e9d26e5541e1ecf2@syzkaller.appspotmail.com>
Acked-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Conflicts:
	kernel/rcu/srcutree.c

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Reviewed-by: Xie XiuQi <xiexiuqi@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>