io_uring: fix null-deref in io_disable_sqo_submit

general protection fault, probably for non-canonical address
	0xdffffc0000000022: 0000 [#1] KASAN: null-ptr-deref
	in range [0x0000000000000110-0x0000000000000117]
RIP: 0010:io_ring_set_wakeup_flag fs/io_uring.c:6929 [inline]
RIP: 0010:io_disable_sqo_submit+0xdb/0x130 fs/io_uring.c:8891
Call Trace:
 io_uring_create fs/io_uring.c:9711 [inline]
 io_uring_setup+0x12b1/0x38e0 fs/io_uring.c:9739
 do_syscall_64+0x2d/0x70 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

io_disable_sqo_submit() might be called before user rings were
allocated, don't do io_ring_set_wakeup_flag() in those cases.

Reported-by: syzbot+ab412638aeb652ded540@syzkaller.appspotmail.com
Fixes: d9d05217 ("io_uring: stop SQPOLL submit on creator's death")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -8895,7 +8895,8 @@ static void io_disable_sqo_submit(struct io_ring_ctx *ctx)
 	mutex_unlock(&ctx->uring_lock);
 
 	/* make sure callers enter the ring to get error */
-	io_ring_set_wakeup_flag(ctx);
+	if (ctx->rings)
+		io_ring_set_wakeup_flag(ctx);
 }
 
 /*