io_uring/zcrx: recheck ifq on shutdown

io_ring_exit_work() checks ifq before shutting it down and guarantees
that the pointer is stable, but instead of relying on rather complicated
synchronisation recheck the ifq pointer inside.

Reported-by: Kees Bakker <kees@ijzerbout.nl>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/905e55c47235ab26377a735294f939f31d00ae53.1739934175.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: isilence

io_uring/zcrx.c

@@ -489,9 +489,9 @@ void io_shutdown_zcrx_ifqs(struct io_ring_ctx *ctx)
 {
 	lockdep_assert_held(&ctx->uring_lock);
 
-	if (ctx->ifq)
-		io_zcrx_scrub(ctx->ifq);
-
+	if (!ctx->ifq)
+		return;
+	io_zcrx_scrub(ctx->ifq);
 	io_close_queue(ctx->ifq);
 }