io_uring: fix racy IOPOLL completions

io_uring: fix racy IOPOLL completions

BugLink: https://bugs.launchpad.net/bugs/1910111
commit 31bff9a51b264df6d144931a6a5f1d6cc815ed4b upstream.

IOPOLL allows buffer remove/provide requests, but they doesn't
synchronise by rules of IOPOLL, namely it have to hold uring_lock.

Cc: <stable@vger.kernel.org> # 5.7+
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>