srcu: Apply *_ONCE() to ->srcu_last_gp_end

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>