io_uring: correct pinned_vm accounting

->mm_account should be released only after we free all registered
buffers, otherwise __io_sqe_buffers_unregister() will see a NULL
->mm_account and skip locked_vm accounting.

Cc: <Stable@vger.kernel.org>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/6d798f65ed4ab8db3664c4d3397d4af16ca98846.1664849932.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/io_uring.c

@@ -2585,12 +2585,6 @@ static void io_req_caches_free(struct io_ring_ctx *ctx)
 static __cold void io_ring_ctx_free(struct io_ring_ctx *ctx)
 {
 	io_sq_thread_finish(ctx);
-
-	if (ctx->mm_account) {
-		mmdrop(ctx->mm_account);
-		ctx->mm_account = NULL;
-	}
-
 	io_rsrc_refs_drop(ctx);
 	/* __io_rsrc_put_work() may need uring_lock to progress, wait w/o it */
 	io_wait_rsrc_data(ctx->buf_data);
@@ -2633,6 +2627,10 @@ static __cold void io_ring_ctx_free(struct io_ring_ctx *ctx)
 	WARN_ON_ONCE(!list_empty(&ctx->ltimeout_list));
 	WARN_ON_ONCE(ctx->notif_slots || ctx->nr_notif_slots);
 
+	if (ctx->mm_account) {
+		mmdrop(ctx->mm_account);
+		ctx->mm_account = NULL;
+	}
 	io_mem_free(ctx->rings);
 	io_mem_free(ctx->sq_sqes);