io_uring: fix waiters missing wake ups

There are reports of mariadb hangs, which is caused by a missing
barrier in the waking code resulting in waiters losing events.

The problem was introduced in a backport
3ab9326f ("io_uring: wake up optimisations"),
and the change restores the barrier present in the original commit
3ab9326f ("io_uring: wake up optimisations")

Reported by: Xan Charbonnet <xan@charbonnet.com>
Fixes: 3ab9326f ("io_uring: wake up optimisations")
Link: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1093243#99


Reviewed-by: Li Zetao <lizetao1@huawei.com>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>